GB News breached impartiality rules in an interview with Jeremy Hunt, the broadcasting watchdog has found.
Philip Davies and Esther McVey, two Tory MPs who host a regular show on the channel, interviewed the chancellor ahead of the Spring Budget in March.
The watchdog said the interview failed to include an "appropriately wide range of significant views".
GB News said it was "disappointed" by the decision, and called Ofcom's definition of impartiality "imprecise".
In its submissions to Ofcom, GB News argued the programme, which included multiple guests, gave "due weight" to other views.
The broadcaster argued the interview was approached "from the perspective of 'the right'" and it allowed for a free debate on a wide range of views.
This is the third breach of Ofcom rules recorded against GB News since it launched in June 2021.
The watchdog has six further investigations open into potential breaches of impartiality on the channel, including two more against shows fronted by husband and wife duo Mr Davies and Ms McVey.
GB News employs several sitting Conservative MPs as hosts, including deputy party chairman Lee Anderson and former cabinet minister Jacob Rees-Mogg.
Ofcom rules ban serving politicians from acting as news presenters, interviewers or reporters on television and radio stations, unless there are exceptional circumstances. However, politicians are allowed to host discussions about current affairs.
In total, 45 complaints were made to Ofcom about the impartiality of the pre-recorded interview of Mr Hunt on Saturday Morning with Esther and Phil, broadcast on 11 March.
The interview with Mr Hunt was subject to "heightened special impartiality requirements" because it came four days before the Spring Statement, Ofcom found.
The spring statement, which sets out the government's economic forecasts and policy for the coming months, is the second most important fiscal event in parliament each year after the annual Budget.
Ofcom found that Mr Hunt's interview, and subsequent debate, overwhelmingly reflected the views of those in the Conservative Party.
"There were only very limited references to wider perspectives on UK economic and fiscal policy in the context," Ofcom added.
Ofcom said the programme did not breach the rules on politicians presenting news programmes, as this programme was classed as current affairs.
Ofcom, which has the power to revoke broadcasting licences, did not suggest any penalty over the impartiality breach.
Responding to Ofcom's latest decision, a GB News spokesperson said the broadcaster "will reflect on Ofcom's view".
They added: "GB News chose to be regulated by Ofcom, and we are proud to play our part in bringing a wider range of opinion to Britain's media landscape.
"We take compliance seriously, and we believe our programme embraced this."