Cameron in £1,000 pension rise pledge: £20 a week increase by 2020 if he wins next Election Prime Minister said he would guarantee a 2.5 per cent rise every year If prices rise quickly the rise for 12million elderly people could be larger The move is an attempt to win the crucial 'grey vote' that Britain's 12 million elderly get a minimum extra 2.5 per cent every year on their state pension from 2015 to 2020 - and more if prices or wages rise faster. By Simon Walters and Glen Owen All pensioners will get a bumper £1,000-a-year rise if the Tories win the Election, David Cameron pledged last night. The Prime Minister said he would guarantee that Britain's 12 million elderly get a minimum extra 2.5 per cent every year on their state pension from 2015 to 2020 - and more if prices or wages rise faster. The offer, which equates to an increase of around £20 a week on the basic state pension, will add about £12 billion a year to the pensions bill. It means that by 2020, the state pension could rise to £160 a week. The move is an attempt to win the crucial "grey vote." 'Security': David Cameron pledged the bumper rise in an attempt to win the 'grey vote' An estimated one in three people who vote in elections are over the official retirement age, both due to longer life expectancy and a significantly higher turnout rate compared with the under-40s. The new package comes after the Prime Minister vowed to maintain the so-called "triple lock" on pension rises. Introduced in 2010, it ensures that the state pension rises each year in line with whichever is highest among average wages, inflation, or 2.5 per cent. It means that though wages rose by only 0.7 per cent last year, the state pension will go up in April by 2.7 per cent - in line with prices. It will take the basic pension, before contribution-related elements, to £113.10 a week. Experts say that keeping the "triple lock" in place until the end of the next Parliament will be worth a bare minimum of £900 a year to pensioners compared with their current income - and potentially much more if wages or inflation race ahead. Mr Cameron has already vowed to introduce a flat-rate pension of £144 a week in 2016. If it rises by 2.5 per cent a year from then until 2020, the weekly pension could be up to £160. Campaigners were quick to point out last night that much of the increase will be swallowed up by rising bills for energy, food and other essential items. An estimated one in three people who vote in elections are over the official retirement age Mr Cameron said: "I am determined to give people who have worked hard and done the right thing security and peace of mind in retirement. We are only able do this because we are taking the difficult decisions to tackle the deficit and put our economy back on the right path. Despite Mr Cameron's new offer, other measures could undermine the standard of living of many pensioners. There has been speculation that better off pensioners could lose benefits such as free bus passes and the winter fuel allowance. The basic state pension now represents a higher share of average earnings than at any time over the past 20 years.