If the constant pinging of her father's mobile phone had become irritating, Jemma Keys chose to ignore it. 

Her father, former Sky Sports presenter Richard Keys, was gently snoring on the floor of their holiday cottage in St Ives as a series of WhatsApp messages popped up on the phone's screen.

Then, the peaceful afternoon reverie was shattered.

'I heard Mum shouting, 'Your father's been having an affair,' ' Jemma reveals today, a recollection that is burned deeply into her memory. 'My heart stopped. Then I asked, 'With who?' '

Already suspicious, Jemma's mother Julia had picked up her husband's phone and discovered a trail of intimate messages from another woman.

Worse, still: the woman was a close friend of Jemma's – a young lawyer called Lucie Rose – less than half 61-year-old Keys's age.

Former Hollyoaks actress Jemma, 33, has never spoken publicly before about the shock, bewilderment and betrayal she felt in that moment in 2016; the instant when her parents' 36-year marriage began to unravel in the most devastating way.

But it was Jemma, perhaps, who felt that betrayal most acutely. 

She not only lost her closest confidante, but arguably the most important relationship of her life – that with her much-adored father.

The terrible revelation was only the beginning of a turbulent, two-year nightmare which plunged Jemma to the depths of her decade-long battle with alcoholism. 

The pain and hurt reached a humiliating crescendo when she began bombarding Lucie with abusive and threatening late-night text messages which referred to her as 'the filthiest sl**' and a 'Qatar w****'.

One warned, ominously: 'I'm watching every move you make. You don't know what's coming.'

Jemma now says she was in a state of 'alcoholic blackout' when she sent them – and would never have done it while sober.

She was dramatically arrested in a morning police raid, and says three officers wearing body armour threw her into a police van in her pyjamas. 

She faced an unseemly court case in the full glare of the media and, potentially, a six-month prison sentence. 

But despite being charged with harassment and appearing before magistrates, she was spared the embarrassment of a full trial when the Crown Prosecution Service (CPS) dropped the case.

Giving her first interview to The Mail on Sunday, she admits she is now 'mortified'.

But the sense of betrayal has taken a far greater toll. 

'All I can say is that it was 18 months down the line from finding out about the affair – which was the most awful betrayal – and I'd hit the lowest, darkest point of my addiction,' she says. 

'The revelation had come so completely out of the blue. I was under the illusion Lucie was the complete opposite of the person she's turned out to be – there was never any inkling she'd do something like this.

'Then we suffered through this emotional rollercoaster: months of false hopes, anger, forgiveness, hurt and pain. I thought she was my friend. I was devastated, distraught and angry.

'Of course, I can't justify what I did. But I felt like my father had been stolen from me, and the affair genuinely has destroyed my family and my relationship with Dad.

'He had been my idol all of my life and, although I still love him, I don't know who he is any more. I have nothing to say to him.'

Much to her credit, Jemma is beginning to move on and is at last dealing with her alcoholism. She says: 'With addiction you lose everything – your friends, your self-esteem. In my heart, I know it's the right time to come out and expose myself and say, I'm ready now. I'm sober and I intend to rebuild my life.

'I've got every excuse under the sun to drink at the moment. But every day I don't is an enormous achievement.'

The family's domestic drama has been awkwardly played out in public thanks to Keys's notoriety. 

He famously parted company with Sky Sports after 20 years in 2011 when off-air sexist jibes he made about a female match official were recorded and leaked.

Since then, he has worked in Qatar as the face of Middle Eastern network beIN Sports, a subsidiary of Al Jazeera. It was there he met Lucie, then 27, who was working for the Qatari Investment Authority.

Since reports of the affair leaked in 2016, they have been photographed together on Qatari beaches, jetting off on holiday, and at a luxury London hotel. Julia has even written a book, The MANscript, about infidelity, in which Keys – for the first time – publicly admitted an affair.

Keys is currently presenting beIN's World Cup coverage, his life largely unchanged. 

But for his family, the poisonous effects of his betrayal are writ large. 'I met up with Dad for the first time in a year a few weeks ago,' Jemma said. 'He said he was really proud of me for getting sober.

'But I'm confused about what Dad is telling me and that has fuelled my anger. The next day I saw pictures of him at Gatwick with Lucie. I'm done with him now.'

Jemma, casual in jeans and fluffy slippers, is astonishingly candid as she lounges on the sofa of her new flat in Sunningdale, Berkshire.

Despite everything, she knows she has come a long way over the past few weeks. She is doing this interview not for publicity, but to be held accountable: to prove to herself, and others, that she can overcome her addiction.

Jemma was sacked by Hollyoaks in 2007 when she got too drunk to make it on to the set, and stints in pantomime and as a runner on American Idol failed to stem her escalating habit.

There were assault charges, including an attack on her mother, and a drink-driving conviction. 'Addiction made me selfish. I thought the world revolved around me, that I was invincible,' she admits. 

'I was aggressive, getting arrested every other week for being drunk on the street. I don't remember a lot of my life. I've lost most of my friends. I've probably been in rehab at least 20 times. There was no trigger for my behaviour – it just kept getting worse.'

It was in this lonely context that, in 2015, she spent three months with her parents in Qatar, where Keys introduced her to the floor manager at beIN's studios, a girl her own age, who invited her to dinner.

'She lived with other British girls, including Lucie, in a Western hotel complex. I thought Lucie was lovely. It was a really girlie, chatty night. The next day, Lucie sent me a text to say how lovely it was to meet me.'

It was the start of a close friendship – but Jemma was only too open to the temptations.

'The girls would have to take me home because I'd get paralytic. I used to black out. The next day I'd have to ask them what I'd done. They couldn't stop me drinking.'

Lucie was already a central part of the social scene and they regularly posted pictures of themselves holding glasses of wine in figure-hugging outfits. 

'We went to bars or had dinner at my parents' house or in restaurants. We were close. I never noticed anything odd in the way Lucie was with Dad.'

Julia, a relationship counsellor, was suspicious before she found the WhatsApp messages. Jemma never saw the messages – her father's white face was enough. Julia has since revealed Lucie had written 'Are you okay?' and 'I wish I was there to give you a big cuddle'.

Jemma said Keys confessed and was more defensive than anything.

'Mum forgave him and thought they could work it out.'

Jemma, however, was furious. 'I was so hurt. I tried to contact Lucie but got her friend. I told her what had happened, and although she was shocked, she wasn't surprised. She told me Lucie had always been 'the Lucie show' and said she was a narcissist.' 

Jemma has seen and contacted Keys only sporadically since then. She has been in and out of rehab; he, too, was in hospital, for a blocked artery. But the spiralling effect on her behaviour was clear. By the time she began sending late-night texts to Lucie, she was drinking three bottles of wine a day from 8am.

There were nine messages sent on October 31 last year; a further three on November 6. One read: 'You're the filthiest sl** I can ever think of. How dare you. F*** me, your parents must be devastated by how you turned out. This is my last message but I've told you to watch your back and I mean it. I'm proud to say I will never be like you.' 

Another mocked: 'Darling, get some plastic surgery because you look like a man. God knows what's underneath there. You Qatar w****.'

Lucie, who was then working for London law firm Baker McKenzie, reported Jemma to the police.

On December 11 at 11am, three officers wearing body armour arrived at the door. 'I was with Mum, still in my pyjamas,' she says.

'They arrested me for harassment, but wouldn't say why. I was shaking – terrified. They had handcuffs which I begged them not to use. Mum was distraught. She kept saying 'What are you doing? She's not well', but they wouldn't tell her what was happening.'

Jemma was driven to a London police station and locked in a cell for hours before being interviewed. She wept when an officer read out the text messages.

'Lucie's statement said she was 'always looking over her shoulder' and feared 'being attacked'. It was so horrendous. I was mortified. I thought, 'What? I said that?' It's not the person I am. I'd never have gone through with any of the threats.'

Jemma was released on bail and, while she admitted sending the texts, pleaded not guilty to harassment at a hearing on December 22, on the grounds that she was unwell when they were sent. 

A trial was set for March. 'It was a dark few months. My drinking was the worst it had ever been. I'd pray every night to wake up in the morning. I thought I was going to die.'

But on March 15, the CPS dismissed the case after receiving new evidence and deciding there was no realistic prospect of conviction.

'It didn't sink in. Obviously I was relieved but there were mixed emotions, still that anger and betrayal.'

It's hardly surprising. Julia is filing for divorce and, having battled a rare form of thyroid cancer, is living in a friend's annexe while she awaits more cancer tests.

The whole affair has left Jemma determined to stay sober, rebuild her career and concentrate on building her relationship with her long-term boyfriend James. But what her future holds with her father is less clear.

'I have nothing to say about Dad and Lucie. Dad has tried but he is not the person I used to know any more. We used to have such a laugh – we were so close. I miss my dad.

'Would I see him today, if he were here? In this moment, I don't have the answer to that.'