The Office for Budget Responsibility estimates the triple lock will cost £15.5billion a year by 2029-30

State pensioners are being warned not to assume the triple lock will last forever, amid growing questions over whether the costly policy can survive.

With pressure mounting on the public finances, some believe it is only a matter of time before the guarantee is scaled back or scrapped altogether.

Millions of state pensioners could receive smaller annual pension rises if Andy Burnham scraps the triple lock after becoming Prime Minister.

The policy guarantees the state pension increases each year by whichever is highest: inflation, average earnings growth or 2.5 per cent.

Nigel Green, chief executive of financial advisory firm deVere Group, said pensioners should treat the debate as a "wake-up call", arguing that Mr Burnham will eventually have little choice but to scrap the triple lock.

The warning comes as Britain faces rising costs from an ageing population, higher NHS spending, increased defence budgets and growing national debt.

Some advisers believed to be close to Mr Burnham have also reportedly questioned whether the policy can be maintained indefinitely.

Mr Green said the triple lock remains politically popular, but warned that economic pressures would eventually force change.

"I don't believe the question is whether it eventually changes. The question is when," he said.

He added: "Britain is getting older, healthcare spending is climbing, defence demands are increasing and debt interest remains elevated. Every Prime Minister and Chancellor will be looking for room to manoeuvre, and the state pension will inevitably be part of that conversation."

According to the Office for Budget Responsibility, the triple lock is expected to cost the Treasury £15.5billion a year by 2029-30.

Spending on the state pension is also forecast to rise from around 5 per cent of GDP today to almost 9 per cent by the 2070s.

Mr Green believes Mr Burnham could keep the triple lock during the next Parliament but would come under growing pressure to rethink it after that.

"Fiscal arithmetic doesn't negotiate," he said. "If the government wants to preserve spending elsewhere without imposing ever-higher taxes, every major spending commitment comes under scrutiny."

He cautioned that no policy guarantee endures indefinitely, with governments adapting as economic circumstances shift.

"Retirement planning built on the expectation that politicians will always preserve today's promises is taking a risk that many people don't fully appreciate," Mr Green added.

The financial adviser urged people to look beyond the state pension when preparing for retirement.

Building varied retirement savings through long-term investment has become increasingly essential, he argued, as financial security now rests more with individuals than governments.

However, not everyone agrees that the triple lock is unaffordable. Dennis Reed, director of Silver Voices, a membership organisation representing senior citizens, has previously dismissed such claims as "nonsense."

Mr Reed argued that spending decisions are fundamentally political choices, and pensioners are being unfairly blamed for Britain's economic difficulties.

"There are billions of pounds of public spending. It's a political choice, whether you actually put that money into giving older people a decent retirement in dignity, or you put it somewhere else," he said.

When asked about alternative funding sources, Mr Reed pointed to large corporations.

"There are plenty of big profits being made by internet giants and banks and energy companies," he noted, adding that it "wears a bit thin" when pensioners receiving £185 weekly are expected to support the economy while such wealth exists elsewhere.