Seize the moment or rue the day, Prime Minister

Cromwell said "You must not only strike while the iron is hot. You must also make the iron hot by striking."

The landslide that occurred last Thursday has changed, more than any of us dared to imagine, the whole landscape of our politics. Britain seems a different place now. The air of expectation of change is almost tangible.

The temptation for opposition parties, as Parliament returns today, is to leave change to the Government and wait for the dust to settle. But this would be to risk losing the moment and its potential.

The Conservatives will be distracted and disabled by their internal squabbles for some time to come. But the rest of us do not have to wait for them.

In our most successful election campaign since the 1920s, the Liberal Democrats have played a key role not only in sweeping out a discredited government, but also in creating a climate for change. We must now be prepared to play our part in that change.

Our successes have given us a clear mandate to fight for investment in education and in our health service, for a new environmental agenda, democratic renewal and for a more rational, less confrontational and more honest approach to politics. These will be our priorities in the new Parliament.

Mr Blair now has the overwhelming good will of the nation behind him. And that will, too, is for change - fundamental change to the way we do things in our politics and in our society. This could be - should be - a government to match those of 1906 and 1945; one which opens the way to a great decade of reform (it cannot be done in less) to modernise the way Britain works: from our system of government, to our relations with our neighbours, to the way we organise our society and welfare system. And if that is what Mr Blair intends to make it, then the Liberal Democrats will provide critical but firm support for every step he takes in that direction.

We could start immediately by moving to a less adversarial and more constructive and rational basis for our politics in the House of Commons. If Mr Blair intends to change the way the Government behaves, then, again, we will respond by seeking to change the way the Opposition behaves, too.

This will not only improve our politics as they are. It will also prepare the way for our politics as I hope they will become.

It is not to diminish Mr Blair's achievement to note that, for all Labour's huge majority, he still enjoys the support of only 44 per cent of voting Britain. A system that can produce a landslide to sweep a government in on a minority of the vote can sweep it out on a minority of the vote, too. We will need more stable foundations than this to sustain the full decade of reform necessary to complete the modernisation of Britain.

The single most crucial decision that Labour and the Liberal Democrats have agreed to implement in this Parliament is a referendum on proportional representation. If this is won and implemented before the next election, the firm foundations will be laid for a historic period of change which begins with Parliament's return today. If not, this could just as easily be merely another bright dawn, as illusory as all those we have seen before.

It will be much easier to win the case for PR in that referendum if the co-operative politics that PR will entrench is already seen to be working and producing results. So first, we must begin without delay the work agreed by Robert Maclennan and Robin Cook before the election on constitutional change; in particular, the early establishment of the Commission on Electoral Reform.

If, as I believe, this can quickly reach a conclusion on an alternative proportional system broadly supported by both parties - but especially by Mr Blair himself - then the relationship between the Liberal Democrats and Labour will be the most important relationship in British politics for the next 10 or 15 years. The foundations upon which a programme for the full modernisation of Britain can safely be built will have been laid. And the Conservatives will have all the time in the world to try to struggle back to decent one-nation Conservatism, in opposition.

If, for lack of personal commitment, courage or competence, the two parties fail in this project, then this will be just another new government, with another new programme. We will become just another Opposition, opposing them. And the Conservatives, armed with a right-wing agenda and under a right-wing leader, will probably - sooner rather than later - sweep back by the very system that has just swept them away.

If, between us, we lose this moment, it may not come again for a generation.

So Liberal Democrats stand ready to play our part in securing real change in this Parliament. We will strenuously oppose those things the Government does that we think are wrong. We will be a scourge for extra investment in education and for facing up to the crisis in the National Health Service. We will support Scottish devolution, as we have always done, but oppose Labour's damaging decision to have a second question on tax-raising powers in their referendum.

We will be implacable in protecting individual liberties against any repeat of Labour's cavalier disregard for these, in the face of the Howard Bills before the election. We will certainly expect early action to see a Bill of Rights incorporated into British law. We will press for a long- term approach and oppose short-term gimmicks, of which we saw too many during the election. We will not let Labour ignore the environment, as they did in their election campaign, and we will not hesitate to push them to get off the fence on Europe and provide them with support against their own Eurosceptics when they do.

There will be other areas where we can support them, too. For instance, we have long believed in strengthening the independence of the Bank of England in order to entrench low inflation, and we will support the Government in this now that Gordon Brown has announced his indication to implement this policy. We will support initiatives to raise education standards, while insisting that these are backed by resources.

And in some areas we should be prepared to break with convention and tradition if these stand in the way of more sensible co-operation. Constitutional reform is one clear case for such an approach. Welfare reform, Northern Ireland, and perhaps even Europe could be others.

Last Thursday's vote delivered a mandate for real change in Britain. We must keep the iron hot by striking it.