GB News breached impartiality rules after Chancellor Jeremy Hunt was interviewed by fellow Tory MPs Esther McVey and Philip Davies, Ofcom has found.
An episode of Saturday Morning With Esther And Phil featured a pre-recorded interview that focused on the government's approach to economic and fiscal policies ahead of the budget, which was announced four days later.
The subjects discussed on 11 March 2023 also included personal and corporation tax, government borrowing, the role of economic forecast in budget-setting, and the cost of living crisis.
The watchdog received 45 complaints from viewers who raised concerns that the programme failed to meet impartiality rules.
Ofcom said its investigation found the programme was "overwhelmingly reflective of the viewpoints of different strands of opinion within the Conservative Party".
The watchdog therefore concluded that the news channel "failed to represent and give due weight to an appropriately wide range of significant views on a matter of major political controversy".
Ofcom added that there were "only very limited references to wider perspectives on UK economic and fiscal policy in the context of the forthcoming budget".
"For example, no real attention was given anywhere in the programme to the viewpoints of politicians, political parties, organisations or individuals that either, for example, criticised, opposed or put forward policy alternatives to the viewpoints given by the three Conservative politicians," the watchdog added.
The watchdog said GB News has accepted that the programme dealt with "a matter of major political controversy" and that "special impartiality rules applied".
Ofcom said in its statement: "GB News should have taken additional steps to ensure that due impartiality was preserved."
The findings mark the third breach of Ofcom's broadcasting rules by GB News since it was launched in June 2021.
The watchdog said it has six further investigations into the channel's compliance with impartiality rules.