The Liberal Democrat manifesto will include a commitment to keep the “triple-lock” protection for state pensions, party leader Tim Farron has announced.

But the Lib Dems would strip wealthier pensioners of the winter fuel allowance, at a cost of up to £300 a year to older people.

Labour has also pledged to retain the triple lock, which guarantees the state pension will rise in line with inflation, average earnings or 2.5 per cent, whichever is the highest.

But there is speculation that the Conservatives will ditch the guarantee after Prime Minister Theresa May declined opportunities to confirm it would feature in her manifesto for the 8 June general election.

Mr Farron said his promise showed Lib Dems would help pensioners when the Conservatives would not.

But he is likely to come under fire over the winter fuel allowance from Labour, which earlier this year promised to keep the benefit, worth between £100 and £300 tax-free a year towards the heating bills of all over-65s, regardless of their wealth.

Lib Dems argue the change will just mean rich pensioners losing “perks they do not need”.

Only those whose pensions put them into the 40 per cent tax bracket would lose the allowance under their plans.

Meanwhile, the triple-lock promise would mean the £122.30 weekly state pension rising to at least £131 by 2020.

Setting out his plans, Mr Farron said: “We are looking again at universal benefits paid to the wealthiest pensioners because money should not be paid to those who do not need it.

“We need to help those in need and on middle incomes and not help the super-rich.

“The Liberal Democrats believe that an important test of a civilised society is the way in which it cares for the elderly.

“We will protect the triple lock, unlike the Conservatives.

“The guiding principle of the pensions system must be to ensure that none are left unable to meet their basic needs for survival and participation in society, and that everyone is treated with the respect and dignity they deserve.

“The message is clear: we will help pensioners and the Conservatives won't.

“A Lib Dem pensions minister introduced the triple-lock guarantee to protect the state pension during coalition.

“We delivered on our manifesto commitment to increase the basic state pension by whatever is highest out of CPI inflation, average earnings or 2.5 per cent.”

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The triple lock was a feature of the Liberal Democrat manifesto in 2010 and a key demand in negotiations on the formation of the coalition government with Conservatives.

Since its introduction in 2010, it has seen pensioner incomes rise faster than average earnings and prompted warnings it will swallow up an ever greater share of state spending.

A cross-party House of Commons select committee report earlier this year described it as “inherently unsustainable” and recommended it should not be continued beyond 2020.