A former Conservative minister has urged Theresa May to scrap the ‘triple lock’ on pensions in her manifesto.

David Willetts said that the policy was a “a poor use of public money” and should be ditched.

The Prime Minister later refused to rule out abandoning the guarantee, which has come under increasing criticism in recent years.

The lock means that the state pension rises by the same as either average earnings, the Consumer Price Index (CPI) measure of interest, or 2.5 per cent, whichever is the highest.

But with inflation rising, politicians have expressed concern about the amount of money the policy could end up costing.

In an article in the Evening Standard, now edited by former chancellor George Osborne, who was famously sacked by Mrs May when she entered No 10 last year, ex-science minister Mr Willetts wrote: “The triple lock is a poor use of public money, not least because it fails to tackle the biggest anxiety for pensioners — funding for social care. I would replace it with a pledge to link pensions to earnings, and a new commitment to fund a lasting solution to our care crisis.”

The triple lock is now costing around £6 billion a year and has become a “totemic” issue, he said.

“Of course it is good news that pensioner incomes have been rising but, once allowing for housing costs, they are now outstripping the incomes of working-age families.

"We are in danger of becoming a country organised around the interests of older people.”

Later Mrs May would not commit to keeping the policy.

Taking questions from workers at the Screwfix distribution centre in Stoke-on-Trent, she said the cost of living had gone up and admitted there would be uncertainty over the next two years as Britain quits the European Union.

She said: "We are seeing inflation, obviously, has gone up slightly as the impact of what's happened to the currency, it's partly about that.

"But there are things that we can do, like on energy prices."

Labour's manifesto does contain a promise to maintain the triple lock on pensions.

Mrs May insisted that pensions would "continue to go up" under the Conservatives.

But she refused to guarantee the future of the lock.

She said: "I've been very clear that pensions will continue to rise under a Conservative government.

"How we calculate that will be set out in our manifesto."

Earlier the Conservative leader revealed that she is a Harry Potter fan during a chat with schoolchildren.

But she was cagey when asked which character from the books she thought she most resembled.

Mrs May told youngsters in Birmingham that she had read all of the wizard adventures, written by JK Rowling.

Discussing books with one young Harry Potter fan, Mrs May said: "I've read them all. They are all very good."

But later in Stoke-on-Trent, she insisted she bore no similarities to any of the characters in the books.

Asked which she was most like, Mrs May replied: "I don't think I am similar to any of the characters in Harry Potter.

"But they are a great read for adults as well as for children."

Meanwhile, a new poll suggests that the Conservatives have seen their lead narrow by three percentage points over Labour.

But Theresa May's party is still out in front on 47 per cent compared to Labour's 33 per cent.

The latest findings by Panelbase sees a one percentage point drop for the Tories and a two percentage point increase for Labour.

The Lib Dems fell one percentage point to 7 per cent, Ukip were no change on 5 per cent and there was a one percentage point rise to 3 per cent for the Green Party.

Respondents also showed support for Labour-backed policies such as the abolition of tuition fees, with 51 per cent in favour and 20 per cent against, and public ownership of railways, with 52 per cent in favour and 20 per cent against.

Conservative-backed policies also received support, including the controversial pledge to reduce net migration to the tens of thousands - supported by 65 per cent of respondents compared to 14 per cent who opposed it.

Panelbase surveyed 1,026 adults living in the UK between May 12 and 15.