Charles Kennedy sought to exploit Tory turmoil on Friday by promising to build on the Liberal Democrats' best ever showing and replace the Conservatives as the de facto opposition.

With the Tories consumed over the next few weeks with their leadership contest, the Liberal Democrat leader pledged to begin the "hard work" to justify what he claimed was public confidence in the party's ability to hold the government to account.

The election had been a "referendum on the opposition", Mr Kennedy said, adding that support at the ballot box for the Liberal Democrats' health and education policies gave the party a mandate to pursue the government over public services.

Speaking at a champagne reception in Westminster yesterday, he said: "The public have made their view clear on that in the vote of confidence they have given us. We must not let them down."

Mr Kennedy more than answered critics who had been certain the Liberal Democrats would lose some of the 46 seats they won in 1997. They had accused him of being too laid back, and failing to build on the successes of his energetic predecessor Sir Paddy Ashdown. Instead, the party won 52 seats - the best result for any third party since 1923 - and 18.8 per cent of the vote.

As people voted tactically to keep out the Tories, the Liberal Democrats now have a Westminster MP in every region of the country for the first time. They stunned the Conservatives by seizing former Tory strongholds such as Guildford, Mid Dorset and Cheadle.

The party also snatched Teignbridge, substantially increased its majority in seats such as Torbay, and saw an increase in their share of the vote in Scotland where they pushed the Tories into fourth place in votes cast, despite worries that being in a coalition with Labour in Holyrood would harm their vote.

Lord Rennard, the party's campaign head, said: "The Tories will be in such a state. They are incapable of providing a coherent opposition."

That inevitably places a question mark over the relationship that Mr Kennedy's predecessor, Sir Paddy, forged with Labour before the last election.

One Liberal Democrat insider admitted: "There will be as much opposition as there is co-operation."

In the Lords, where Lord Rodgers of Quarry Bank is tipped to be replaced by the tougher Lord McNally as the party's leader, the party is expected to be especially hard on the government if it fails to make progress reforming public services.

Mr Kennedy will hope to keep open the possibility of further co-operation by virtue of his non-confrontational style. Lord Rennard said that it was not Mr Kennedy's style to indulge in "yah-boo negative opposition".

"It's much more effective to criticise the government when they are wrong but continue to support them when we agree with them," he said.

The Liberal Democrats will aim to put most pressure on the government over the euro, pressuring Tony Blair to hold a referendum sooner rather than later.

Although tactical voting enabled the Liberal Democrats to overcome the disadvantages of the first-past-the-post system at this election, the party will not give up trying to persuade Labour to reform voting procedures. As Lord Rennard put it: "We fundamentally do not think it's right that the Labour party should be able to get two-thirds of the seats with not much more than 40 per cent of the vote."

Whether or not the two parties' joint cabinet committee continues to meet remains to be seen. With its little changed majority, Labour has less need of the Liberal Democrats. Tony Blair had wanted to keep his options open on co-operation between the two parties in case he at some stage needed to combat a Tory resurgence by forming a centre-left alliance. Now that Labour has been returned to power on another landslide, the Liberal Democrats are in some respects more irrelevant to the government.