Kwasi Kwarteng has refused to rule out a new era of austerity as Universal Credit claimants brace for a real-terms cut in benefits.
Fresh from a humiliating U-turn on the mini-Budget, the Chancellor faced accusations of leaving vital services with a "massive black hole in their budgets".
It comes after Mr Kwarteng unveiled over £40 billion in tax cuts - funded by borrowing - without consulting the Office for Budget Responsibility.
Pressed on BBC Radio 4's Today programme whether there would be a "new era of austerity" to fund the measures, Mr Kwarteng declined to rule out the prospect.
"You will see what our spending plans are in the medium-term fiscal plan but I'm not going to be drawn on that," the Chancellor replied.
Asked again whether he would curb spending or drop the multi-billion pound tax cuts, the Chancellor said the government will stick to the 2021 spending review.
Without further funds, government departments could face real-terms cuts of around £18 billion - due to soaring inflation.
In response, Frances O'Grady, the general secretary of the Trade Union Congress, said: "The Prime Minister promised during her leadership campaign there would be no return to austerity. But that pledge now appears to be in tatters.
"If spending on public services does not rise in line with inflation, schools, hospitals and other vital services face a massive black hole in their budgets."
She added: "Public services have already been cut to the bone by successive Conservative governments. There is nothing to trim.
"If Liz Truss pushes ahead with another round of savage cuts this would be a huge betrayal of the British people and proof that her word counts for nothing."
During interviews on Monday, the Chancellor also refused to say whether benefits will be uprated in line with September's inflation of around 10 per cent.
His predecessor at the Treasury, Rishi Sunak, had committed earlier this year to uprating Universal Credit by inflation in April 2023.
Instead, The Sun reported, the benefit could match the annual rate of pay growth of just over 5%.
A government source - referring to energy bills help for those on Universal Credit - told the newspaper: "Claimants have been given a lot of support already so that should be factored in".
