BBC Radio 4 broke accuracy rules by failing to sufficiently challenge the climate change denier Nigel Lawson’s controversial claims in an interview, the broadcasting watchdog has ruled.

Lord Lawson appeared on a Radio 4 programme last summer denying the concept of climate change, which prompted complaints from the Green party and the prominent scientists Brian Cox and Jim Al-Khalili, who said it was “irresponsible and highly misleading” to imply there was still a debate around the science supporting it.

Brian Cox (@ProfBrianCox) I agree with @jimalkhalili . Irresponsible and highly misleading to give the impression that there is a meaningful debate about the science. https://t.co/HtqJf9sBFW

The Today programme featured five interviews on climate change prompted by the release of the film An Inconvenient Sequel, the former US vice-president Al Gore’s follow-up to 2006’s An Inconvenient Truth. Each interview was conducted by the presenter Justin Webb.

Lawson, a former chancellor of the exchequer in Margaret Thatcher’s government, made claims including that “all the experts say there hasn’t been” an increase in extreme weather events. He said the Intergovernmental Panel on Climate Change “concedes” this, and that, according to official figures, “during this past 10 years … average world temperature has slightly declined”.

Ofcom received two complaints that the interview broke the UK broadcasting rule 5.1, which states that “news, in whatever form, must be reported with due accuracy and presented with due impartiality”.

“Neither statement was correct, or sufficiently challenged during the interview or subsequently during the programme,” said the Ofcom ruling.

The BBC said it had publicly acknowledged that “some of Lord Lawson’s statements went beyond the intended scope of the interview and he was allowed to make inaccurate assertions which should have been challenged”.

Ofcom was not impressed that a previous appearance on the Today programme in 2014 by Lawson, who founded the Global Warming Policy Foundation, resulted in an internal BBC investigation and ruling that found the same failure to properly challenge his views.

BBC apologises over interview with climate denier Lord Lawson Read more

“We found that statements made about the science of climate change were not challenged sufficiently during this interview, which meant the programme was not duly accurate,” said a spokeswoman for Ofcom. “We’ve told the BBC we are concerned that this was the second incident of this nature, and on the same programme.”

The BBC attempted to minimise the fallout of Webb’s failure to address the inaccurate comments at the time, by airing a follow-up item the next morning that “examined some of the more contentious claims made by Gore and Lord Lawson”.

The BBC also published a news story online that “highlighted criticisms of the interview with Lord Lawson and identified the inaccuracy of certain aspects of his contribution”.

Ofcom said these actions saved the BBC from a second breach of the broadcasting rules. Rule 5.2 states that “significant mistakes in news should normally be acknowledged and corrected on air quickly … corrections should be appropriately scheduled”.

The BBC also included contributions from several complainants and a statement from the Today programme in Radio 4’s Feedback slot 10 days after the original interview.

The BBC decided against pitting Lawson against a scientist who could have challenged his views during the interview. The corporation said this was deliberate, because it had done so in 2014 and the tactic had backfired, legitimising Lawson “by giving listeners the impression of parity between their views”.

The decision not to have an expert on hand to challenge Lawson meant listeners were “not given the full facts” about his stance on climate change, and meant Webb should have been prepared to step in and correct him, Ofcom said.

“The editorial team could have reasonably anticipated there was a risk that Lord Lawson might raise these arguments,” said Ofcom. “The BBC should have planned for that eventuality and the presenter should have been prepared to provide challenge and context to Lord Lawson’s views as appropriate.

“The BBC’s failure to do this led to significant inaccuracies being broadcast. Critically, these inaccuracies were allowed to stand without challenge or clarification during that broadcast.”