LONDON (REUTERS) - Britain's Prime Minister David Cameron promised on Sunday to protect pensioners' income if his Conservative party wins the 2015 election, laying out his first manifesto spending pledge.

Coming at a time when Britain is facing years of cuts to public spending, the promise is designed to win the support of the country's elderly - who polling data show are more likely to vote in elections than younger people.

Mr Cameron said he would extend the current 'triple lock' on pension payments, which ensures they rise at the pace of either inflation or wage increases - whichever rate is higher - and by a minimum of 2.5 per cent per year.

"In a civilised society ... knowing you're going to have a decent state pension ... is, I think, a really powerful thing," Mr Cameron said in an interview with the Sunday Times newspaper. He said the policy was the "first plank" of his election manifesto.

The existing triple-lock system was agreed in 2010 with the Liberal Democrat Party, the junior partner in Britain's coalition government, and is also supported by the Labour party, but neither have yet pledged to continue it after 2015.

In 2012/13, total spending on the state pension and pensioner benefits was seen at 110 billion pounds (S$ 220 billion), or 7.1 per cent of the country's gross domestic product, official projections showed.

To win the next election, MR Cameron needs to wipe out the 7 percentage point opinion poll lead of the centre-left opposition Labour party.

Labour has been campaigning on a promise to address what it calls a "cost of living crisis", criticising the government for failing to deal with stagnant wages and rising prices. A poll commissioned by former Conservative Chairman Michael Ashcroft published on Saturday said Cameron's party had lost the support of a third of those who voted for it in 2010.

As Britain's population ages, Mr Cameron has faced pressure to pare back access to benefits such as winter fuel subsidies and free prescription medication that are provided to all pensioners regardless of their income.

However, he said that the "difficult decision" announced last month to raise the retirement age meant he could afford to extend the basic pension protection.

The age at which Britons will become eligible for the state pension is to rise by one year to 66 by 2020 and will be as high as 68 by the mid-2030s.