LIZ Truss has effectively staked what remains of her credibility on increasing pensions by more than 10 per cent after telling MPs she is “completely committed” to the triple-lock.

However she failed to make a similar promise about uprating other benefits in line with inflation next April.

Ms Truss’s assurance on the triple-lock came after inflation rose to a 40-year high of 10.1% in the year to September, the month used as the benchmark for pension increases.

The triple-lock commits the Government to raising the state pension by the highest of inflation, average earnings or 2.5%, and would cost £11billion to deliver next year.

No 10 yesterday briefed the media that Ms Truss was no longer committed to the triple-lock, with her spokesman saying she was “not making any comments” on spending, sparking a backlash from Tory MPs.

But at PMQs, in response to a question from SNP Westminster leader Ian Blackford, Ms Truss insisted she was sticking with the triple lock, as she had promised two weeks ago.

If she were to U-turn on pensions, it could prove the last straw for Tory MPs already exasperated by her premiership.

Taunted by Sir Keir Starmer about being the impotent head of an “opposition in waiting” after her economic plans were shredded by Chancellor Jeremy Hunt, Ms Truss also insisted she was staying on, telling him: “I am a fighter and not a quitter”.

The phrase was famously used by New Labour’s Peter Mandelson in 2001 after he was re-elected as the MP for Hartlepool despite a recent scandal.

Ms Truss also opened the session with an apology, telling MPs: “I have been very clear that I am sorry and that I have made mistakes.” 

Mr Blackford said: “After ten U-turns in two weeks, we're left with a Prime Minister in office but not in power, and families are paying through the teeth for her mistakes. 

“Her latest broken promise has put pensioners in the front line of Tory cuts. 

“So can the Prime Minister perhaps turn to her Chancellor right now, get permission to make another U-turn, and commit to raising the state pension at the rate of inflation?”

Ms Truss replied: “I honestly don’t know what the honorable gentleman is talking about. We have been clear in our manifesto that we will maintain the triple lock and I am completely committed  to it. So is the Chancellor.”

After Mr Blackford continued with a scripted question about Ms Truss throwing millions of pensioners “under the bus”, Ms Truss said he was unable to take Yes for an answer.

She said: “I’ve been clear. We are protecting the triple lock on pensions.”

At last week’s PMQs, Ms Truss said she was “absolutely” sticking by her Tory leadership campaign pledge not to cut public spending.

However that lasted only a few days, until she sacked Kwasi Kwarteng as Chancellor and installled Mr Hunt, who has said there will have to be spending cuts.

Downing Street said the decision was taken “jointly” by Ms Truss and Mr Hunt on Wednesday morning ahead of her appearance at the despatch box.

The Prime Minister’s official spokesman said it reflected the “unique position” of pensioners who are unable to increase their income through work.

“She and the Chancellor have discussed and agreed the position the Prime Minister set out this morning,” the spokesman said.

But Ms Truss faces a rebellion on another front after she refused to give the same commitment to increasing benefits in line with inflation.

After Tory MP John Baron urged her to show “compassion in politics” by maintaining the inflation link, she was conspicuously non-committal.

She said: “We are compassionate Conservatives, we will always work to protect the most vulnerable.”

The triple lock was temporarily suspended for 2022/23 because earnings soared as people returned to work from furlough and coronavirus restrictions, but was expected to be restored before inflation soared.

Existing policy dictates that, from April next year, the state pension and benefits should increase by 10.1%, the figure for Consumer Price Index measure of inflation in the preceding September.

Tory critics have threatened to rebel if Ms Truss backtracks on the welfare payments commitments, which would hit claimants with real-terms cuts.

Liberal Democrat work and pensions spokeswoman Wendy Chamberlain said the Government had been “dragged kicking and screaming by their own backbench’s backlash into doing the right thing”.

“This is a party in utter chaos, that can’t keep its own policies straight from day to day, let alone through the winter,” she added.

Age UK director Caroline Abrahams said the rate of inflation “only strengthens the case for reinstating the triple lock” and warned Ms Truss that breaking the promise would be “devastating for the millions of older people who rely on the state pension”.

She said: “Knowing their state pension would keep pace with rising prices because of the triple lock has given precious hope to many older people at a time of great anxiety. For the Government to take that away from them now would be a hammer blow, as well as a flagrant breach of trust.”

The Institute for Fiscal Studies has estimated that each percentage point increase in inflation will add £2.4bn a year in pension and welfare spending from April.

The Office for Budget Responsibility had estimated a 7.5% inflation level, so the 10.1% figure add billions to plans for the welfare bill.

The Chancellor is expected to set out his plans for pensions and benefits in his October 31 statement, alongside spending cuts.