The party plans to "guarantee in law" the "triple lock" arrangement whereby each year pensions would rise by the highest of wages, prices or 2.5 per cent if elected at the 2015 General Election. The state pension would be worth at least £131 per week by 2020, from £97.65 four years ago, Mr Webb said.
He added the basic state pension had increased in each year of the last Parliament with the level now £440 higher per year in 2014-15, than if pensions had risen in line with earnings, worth £800 a year more in total.
Pensions Minister Steve Webb, who says it will be in the party's manifesto, added: "A decent income in retirement for pensioners is central to the Liberal Democrat vision of a fair society.
"For decades, successive Labour and Conservative governments allowed the state pension to decline after Margaret Thatcher broke the 'earnings link' in 1980.
"Our manifesto promise of a 'triple lock' has been implemented every year since 2010 and means that the state pension is already a higher share of the national average wage than at any time since the early 1990s.
"But if we are serious about having a decent state pension we need to go further. That is why the Liberal Democrats will guarantee in law that in each year pensions will rise by the highest of wages, prices or 2.5 per cent."