Media playback is unsupported on your device Media caption David Cameron: "I want people to know that they can have dignity and security in their old age"

The state pension will continue to rise by at least 2.5% a year until 2020 if the Conservatives win the next election, David Cameron has said.

The PM pledged to keep the "triple lock" system, which ensures the state pension goes up by whichever is higher - inflation, wages or 2.5%.

He said it was "fair" to prioritise pensions even at a time when benefits for younger people were being slashed.

Labour said it supported the triple lock "in principle".

'Dignity and security'

Mr Cameron has described the "triple lock" announcement as the "first plank" of the Conservative general election manifesto.

But in an interview with the BBC's Andrew Marr Show, he rejected suggestions it was a purely political move aimed at attracting older voters back to the Tory fold.

Analysis All politicians know there are votes in appealing to the older generation. That is why Labour and the Lib Dems are unlikely to oppose David Cameron's state pension "guarantee" despite concerns it will increase the transfer of wealth in the UK from young to old. But Mr Cameron may have a particular set of more mature voters in mind with his "triple lock" announcement. A new poll by Lord Ashcroft suggests Tory voters are deserting the party for UKIP in even greater numbers than previously thought. Research last year by YouGov suggested UKIP draws the majority of its support from older people - far more so than the traditional "big three" parties. This is one way for Mr Cameron to tempt them back to the fold without getting involved in potentially bruising rows with Nigel Farage about immigration and the EU.

"It's a choice based on values, based on my values." he told Andrew Marr.

"I want people when they reach retirement to know that they can have dignity and security in their old age.

"People who have worked hard, who have done the right thing, who have provided for their families, they should then know they will get a decent state pension and they don't have to worry about it lagging behind prices or earnings and I think that's the right choice for the country."

He said the government also had to do more to help young people such as increasing the number of apprenticeships.

He refused to drawn on whether the Conservatives would cut back on pensioner benefits such as the winter fuel payment, free bus passes and television licences for the over 75s.

The Lib Dems and Labour have both supported the pensions "triple lock" in principle but have made no commitment about whether they would keep it after the next election.

Labour leader Ed Miliband said: "We will set out our plans in the manifesto for all of our tax and spending proposals.

"That's the right time to do it but nobody should be in any doubt about my commitment to the triple-lock on pensions."

Asked whether the triple lock would be included in any welfare cap, he said: "In the short term we've said they're not part of the social security cap. Obviously in the longer term we have to keep an eye on these things, the long term forecasts for pensions."

'Gravy first'

Mr Miliband said he was more concerned about David Cameron's hint, in an interview with the BBC's Andrew Marr, that he was hoping to be able to offer tax cuts as the economy improved and left open the door to a further reduction in the top rate of income tax.

Media playback is unsupported on your device Media caption Ed Miliband: "Nobody should be in any doubt about my commitment to the triple-lock on pensions"

"So he wants further tax cuts for the richest in our society at a time when ordinary families are facing a cost of living crisis," added Mr Miliband.

Former Labour minister and long-standing poverty campaigner Frank Field warned there would have to be tax increases to pay for any "triple-lock" pledge and asked why "yet again pensioners should be exempt when everybody else is being called on, quite rightly, to make sacrifices".

He told the BBC News Channel: "Younger families, with children, who are hungry deserve a similar amount of dignity, particularly if they are working.

"If you are promising one group - a group that is more certain to vote - that they will get the gravy first, you are saying to others there is less for you... and somebody is going to have to pay the taxes to foot that bill."

The triple lock was introduced by the coalition and means many pensions have risen by about £15 per week since 2010.

Chancellor George Osborne announced in his Autumn Statement that the state pension age would increase to 68 in the mid-2030s and to 69 in the late 2040s.

It will rise to 66 by 2020 and to 67 between 2026 and 2028.

Mr Cameron has previously faced criticism - including from senior Conservative cabinet members - for sticking to a 2010 election promise not to cut benefits for the elderly.

But Paul Green from Saga, which specialises in products and services for the over-50s, said: "David Cameron's commitment to the triple lock for the state pension will be warmly welcomed by British pensioners, giving them confidence that their lifetime of work will be properly valued by society."

The triple lock has already helped protect pensioners' incomes at a time when earnings growth has been low.

As a result, the basic state pension will be about £440 a year higher from next April than it would have been if it had risen in line with average earnings since 2011-12.

The triple lock meant the basic state pension rose by 5.2% in 2012, or £5.30 a week - the largest cash rise ever seen.

In April 2013, it rose by £2.70 to £110.15 a week - a rise of 2.5%, which was higher than either earnings or inflation.