Last weekend, Gordon Brown set out his stall, claiming that Labour is the party of fairness in Britain. Their General Election slogan, "A Future Fair for All", seeks to stake out this territory as their own. But, after 13 years in government, are these claims credible? Gordon Brown also urged people to "take a second look" at Labour. What, then, is Labour's record on fairness really like?

The slogan itself is old, first used back in 2003. Then, like now, the poorest paid a higher proportion of their income in tax than the richest. But there is a difference: then, the gap was about two percentage points. Now, it is about four percentage points.

In other words, the gap between what the rich and poor pay in tax has doubled. This is a staggering change. If anyone deserved tax cuts in the last seven years, surely it was those who were finding it hardest to make ends meet, not those at the top?

When I asked Gordon Brown about this yesterday at Prime Minister's Questions, he avoided the question. He said I'd forgotten tax credits. But what he knows perfectly well is that those damning figures include the effect of tax credits, penny for penny, pound for pound.

Brown is hiding the truth that, even after every last tax credit has been handed out, the poorest are still the ones hit hardest. Just imagine how it feels to struggle so hard just to make ends meet knowing such a huge chunk of your meagre income goes straight back to Treasury coffers.

Labour has no basis on which to claim the ground of fairness any longer. Their record is one of rising inequality and unfairness. The roll-call of failures is long. Income and wealth inequality have not improved a jot in 13 years of Labour. A child born in a poor part of Sheffield will die 14 years before a child born just up the road. Even the brightest children from deprived backgrounds are likely to fall behind their more affluent classmates at school before their seventh birthday.

Thousands of children are in classes so big they're technically illegal. Carbon emissions are higher than when they came to power, a devastating example of inter-generational unfairness. Our political system can be bought and sold by big donors. Only in Gordon Brown's bunker can any of this be considered fair.

Based on what has happened since 2003, Labour's words are not a slogan at all. They are a warning. Even when all the sound and fury over the allegations of bullying in Downing Street which have dominated the headlines this week have died down, this legacy of unfairness will remain.

This is not a question of character, it is a question of values and priorities. Labour has failed on fairness. Only far-reaching change can make the future fair as it should be. All those millions of voters who flocked to Labour in 1997 with great hope for a fairer future must not despair, but look elsewhere for a political home.

I believe there is a way to build the fair future that so many of us dream of, but it means doing things very differently. There's lots of flaky talk from the Conservatives about the need for change. Flaky because on so many issues it is impossible to see what convictions or changes the Conservatives really care about.

And the changes they do want to introduce, like inheritance tax cuts for millionaires or tax penalties for working married couples, will simply exacerbate unfairness in Britain. There's no point advocating change if the effect is to turn the clock back.

I believe there are four big areas where fairness should be hardwired into Britain: into the tax system, by introducing a £10,000 tax-free allowance for everyone, paid for by closing the loopholes for the very rich; fairness in the classroom with smaller class sizes; a fairer economy where growth and jobs are spread across the whole nation, not concentrated in the City; and fairer, open politics, starting with wholesale reform of the murky way parties are funded and the way MPs are held to account.

I am often asked to predict the outcome of the general election or exactly how the Liberal Democrats will react in an array of different possible outcomes. No one, of course, can predict the future. But I can guarantee what our priorities and principles will be in all circumstances: fairness in taxation; fairness in childhood; fairness in our economy; fairness in politics. These are the ambitions that animate my party, and will be at the heart of our election campaign.

Labour was once a credible home for those who hoped for a better future. But they are like a spent match. There is nothing left. The centre of the progressive movement in British politics has shifted: the Liberal Democrats are the real party of fairness now.

The writer is the leader of the Liberal Democrat Party