The prime minister has vowed to guarantee annual increases in the state pension until 2020, and marked them out as the only welfare spending to be exempt from the forthcoming cap if the Conservatives win the next election.

David Cameron's pledge aims to reassure older voters that their payouts will continue to rise in line with inflation despite a squeeze on other benefits such as winter fuel payments, TV licences, and free bus passes and prescriptions.

"We can only afford to do this because we are taking difficult decisions to cut the deficit and get spending under control as part of our long-term economic plan. I want to do everything we can to help people who have worked hard and done the right thing," he said.

Under the new proposals, the basic state pension will be around £440 a year higher from April than if it had been increased in line with average earnings since 2011-12, Downing Street said. Cameron told the Sunday Times that the pension pledge was "the first plank of the next general election manifesto".

"In a civilised society … knowing you're going to have a decent state pension … is, I think, a really powerful thing," he added.

Defending the decision to raise the pension age, he said: "People aren't stupid – they know that you can only pay a decent state pension if you take tough, difficult, long-term decisions."

The chancellor, George Osborne, warned in his autumn statement that people will have to wait longer before receiving their state pension. Millions of people now in their 30s and 40s will have to wait until they are 68 or 69 before qualifying. The announcement resulted in trade union protests that young people were being expected to "work until they drop".