The BBC has formally rejected complaints that its Newsnight programme Photoshopped an image of Jeremy Corbyn to make him look “more Russian”, insisting that the programme’s use of the picture was “impartial and fair”.

The corporation’s complaints unit said the decision to show a photo of the Labourleader wearing a “Lenin-style cap” in front of the Moscow skyline was not designed to convey an impression of pro-Russian sympathy on Corbyn’s part but was in keeping with an editorial decision made on the basis of “sound news judgment”.

Video of the alleged Photoshopping incident attracted millions of views following the Newsnight episode broadcast on 15 March, in which the programme discussed Corbyn’s reaction to the poisoning of Sergei and Yulia Skripal in Salisbury and alleged involvement of the Russian state.

Viral footage of the graphic prompted 48 people to formally complain to the BBC that the decision to use the image of Corbyn wearing a cap represented anti-Labour bias on behalf of Newsnight, that the image had been manipulated to emphasise the Russian elements of this hat, and that this bias had been compounded by the decision to include a stylised Russian backdrop.

The BBC’s complaints unit rejected this and ruled the main reason Corbyn’s cap looked more like a Soviet-style fur hat was that the image had been projected on to a large curved screen in the TV studio.

The ruling admitted that some members of the public illustrated their complaints with copies of the original photograph next to a screengrab of the equivalent image in the programme, in which the hat did appear to be slightly taller.

“This, however, was not the result of Photoshopping or otherwise manipulating the image. It resulted from the fact that the screen on to which the image was projected is curved, meaning that the image as a whole appeared higher in relation to its width than it would on a flat surface.”

The ruling said it was immediately apparent to viewers that the contrast and colour balance of the photo had been adjusted to create a stylised image but this was standard practice on the programme.

“Newsnight’s graphics team regularly treats images of politicians from all parties, and others in this way, to create a strong studio backdrop for whichever story is being covered.”

The ruling also defended the decision to portray Corbyn against a Moscow skyline: “Visual montage is a commonly used device in TV programmes to highlight a story or theme. The use of the technique in news programmes such as Newsnight is intended to epitomise the story rather than to express or invite a particular attitude to it, and the montage used in the item in question was no exception.”

“As the focus of the 15 March item was on Mr Corbyn’s reaction to the claim that Russia was responsible for the nerve agent attack, it was entirely apt for the backdrop to combine his image with this backdrop.”

The ruling concluded that there were “no grounds for regarding the contents of the item as less than impartial or fair to Mr Corbyn”.