The BBC has come under fire over plans to air a full recitation of Enoch Powell’s infamous “rivers of blood” speech, with a Labour peer writing to Ofcom about the broadcast and one contributor saying she regrets taking part.

The 50th anniversary broadcast on Saturday will feature the actor Ian McDiarmid reading the full text of Powell’s incendiary speech, interspersed with analysis and criticism. If it goes ahead it will be the first time the speech has been broadcast in full on British radio.

The BBC media editor, Amol Rajan, who will present the programme, said on Twitter on Thursday: “On Saturday, for 1st time EVER, Enoch Powell’s Rivers of Blood speech will be read in full on UK radio.” 

After the announcement, the University of Wolverhampton academic Dr Shirin Hirsch, a contributor to the programme, said she was “disgusted” by how it was being promoted and she now viewed her participation as a mistake.

The Labour peer Andrew Adonis said he had written to the Ofcom chief executive, Sharon White, calling the decision to broadcast the speech “extraordinary” and asking her to instruct the BBC to cancel the programme.

His request was unlikely to be met because Ofcom is a post-transmission regulator. It also advises viewers to complain to the BBC in the first instance.

A statement from the watchdog said: “Ofcom’s powers, granted by parliament, are as a post-broadcast regulator. This means that we wouldn’t check or approve any broadcaster’s editorial content before transmission.”

In his letter, Adonis wrote: “The BBC claims in its advance publicity that this is some kind of artistic enterprise. This argument is unsustainable, particularly in context of the BBC’s boast that the broadcast provides a unique opportunity to hear the speech in full.”

He went on: “As a special tribute to the 50th anniversary of ‘rivers of blood’, the BBC is broadcasting the full text of the most incendiary racist speech of modern Britain that was not even broadcast at the time.”

Rajan, who will present the broadcast on Radio 4 on Saturday night, defended the decision, pointing out that rather than being read continuously, the speech would be broken up and critiqued by a range of voices throughout the programme.

Since only a short section of Powell’s 1968 speech was recorded, McDiarmid will read the full text for the broadcast. He that included the speech in Edinburgh last year.

Powell’s 45-minute speech, delivered to local Conservative party members in Birmingham a few days before the second reading of the 1968 race relations bill, was applauded by attendees and divided the nation.

Attacking the government’s immigration policy as “mad”, Powell called for a reduction in the number of arrivals and a policy to encourage those already in the UK to return to their country of origin. He compared enacting the bill with “throwing a match on gunpowder”.

The speech ended with a reference to a moment in Virgil’s Aeneid when a prophetess predicts civil war in Italy with “the river Tiber foaming with much blood”.

The speech led to Powell being sacked as shadow defence spokesman by the Conservative party’s then leader, Edward Heath.

Thousands of workers staged strikes and marches in support of Powell’s views. In 1974 he left the Conservative party in opposition to Heath’s backing for Britain’s membership of the European Community. Powell died in 1998 at the age of 85.

The BBC said in a statement: “Many people know of this controversial speech but few have heard it beyond soundbites. Radio 4’s well established programme Archive on 4 reflects in detail on historical events and, in order to assess the speech fully and its impact on the immigration debate, it will be analysed by a wide range of contributors including many anti-racism campaigners.

“This is a rigorous journalistic analysis of a historical political speech. It’s not an endorsement of the controversial views and people should wait to hear the programme before they judge it.”

• This article was amended on 13 April 2018 to correct Enoch Powell’s age from 95 to 85, and to correct the reason he left the Conservative party in 1974. An earlier version said this was over “Heath’s plan to join the European Union”.