Ofcom is to investigate whether GB News breached broadcasting rules with a second showing of its interview with Donald Trump after complaints that the US president’s claims about climate change, Islam and immigration had gone unchallenged.

A series of complaints were made over the interview, which the presenter Bev Turner conducted last November.

The media regulator had previously announced that it would not open an investigation into the original broadcast of the interview on the rightwing network’s US-based programme Late Show Live.

In what is emerging as a test case in its approach to impartiality, however, it has now announced that it will investigate a November edition of The Weekend, a GB News show that repeated the interview in full the next day.

Trump was not challenged as he claimed human-induced climate change was a hoax and that London had no-go areas for police. He said parts of the capital had sharia law.

“This programme featured an interview by GB News presenter, Bev Turner, with US president Donald Trump,” an Ofcom spokesperson said. “We are investigating whether it breached our rules on due impartiality and material misleadingness.”

Ofcom has not said why it has opened an investigation into the interview’s second showing and not the first, but it takes into account the content around an interview – such as panel discussions referring to it – as well as other context.

The Weekend was broadcast during the day in the UK, so its audience would have been higher than for the original showing of the interview, which was shown overnight.

The Guardian understands that some groups concerned about the interview’s partiality had been examining a potential legal challenge to Ofcom’s original decision against investigating it.

Ofcom’s decision comes after the departure of Michael Grade as its chair, though his successor, the former Channel 4 chair Ian Cheshire, has not yet formally taken up the role.

Richard Wilson, the director of the Reliable Media campaign group and a complainant about The Weekend broadcast, said the investigation had taken too long to come.

“Ofcom has quietly opened an investigation six months after the programme aired,” he said. “In that time, GB News’s social media clips of Trump claiming climate change is a ‘hoax’ have clocked up over a hundred thousand engagements online.

“This is what regulatory failure looks like. Today’s announcement is welcome, but it is a direct result of sustained pressure from the public, from MPs and from civil society.

“The new Ofcom chair has inherited a dysfunctional regulator, and parliament must ensure he is held to account for fixing it.”

GB News said it was “surprised and concerned” by what it described as Ofcom’s “delayed decision” over the Trump interview, pointing to the regulator’s previous decision not to pursue complaints about its original airing.

“Ofcom’s U-turn over the repeat of the interview with the US president, Donald Trump, follows adverse commentary around its original decision by prominent critics of both Ofcom and GB News,” it said.

“The sequence of events inevitably raises questions around the rationale for reopening the matter at this stage. It also raises serious concerns around regulatory certainty, procedural fairness and the consistency of Ofcom’s processes.

“GB News stands firmly by its journalism and editorial standards.”