The extraordinary surge in support for the Liberal Democrats continued to gather pace today as a Guardian/ICM poll put the party in second place on 30%, up ten points in a week.

Labour has been forced into third for the first time ever in the Guardian/ICM voting series, which began in 1984.

The findings – which follow a dramatic series of weekend polls including an ICM survey only partly carried out after last Thursday's televised leaders' debate – suggest Britain is on course for a hung parliament.

Since the last Guardian/ICM survey a week ago, Labour support has sunk by three points and Conservative support by four points.

All the fieldwork for the poll was carried out after the leaders' debate.

The Conservatives are on 33%, the score achieved under Michael Howard in the 2005 election and the party's lowest in any ICM poll since July 2007. Labour, on 28%, is at its lowest since last October.

Other smaller parties are on 9%, including four for the nationalists, two for Ukip and one for the Greens. There is no sign of the Lib Dem surge also helping other small parties.

Compared with the more recent weekend ICM poll for the Sunday Telegraph, which was conducted only partially after the debate, Labour are down one, the Conservatives down one and the Lib Dems up three.

It is impossible to calculate today's poll results reliably in terms of parliamentary seats although, if votes moved proportionately in every seat, Labour might end up as the largest party despite coming third in votes.

The Lib Dems, although second in votes, would be substantially behind the other two parties in terms of seats.

The party is attempting to manage expectations after its recent poll boosts.

Speaking in London, the Lib Dem leader, Nick Clegg, said: "I think this general election campaign is starting to come to life for the simple reason that a growing number of people are starting – it is only a start – starting to believe, starting to hope, that we can do something different this time.

"[They are starting to believe] that the old tired choices that they have been given by the old parties of the past no longer need to govern the way in which we run politics in the future."

The Tory leader, David Cameron, warned voters that a hung parliament would result in a Labour government, saying it would lead to politicians "haggling" and trying to "stitch things together" while voters could find they were "stuck with what you have got".

"We have to explain that actually if you get that, you are not going to get the decisive action and the change we need," he said at a campaign event in London.

The Tories today shelved a party political broadcast attacking Labour in favour of one that "responds to the public mood".

The broadcast, due to have been aired this evening, has been replaced with a "personal statement" from Cameron in which he details his values and qualities. The statement was filmed last night in the back garden of his London home.

Gordon Brown said the election should be decided on issues, not personalities. Speaking to journalists on a train to the West Midlands, he said: "We have heard quite a lot about who people are, but not what they stand for.

"It's not going to be the X Factor multiplied by three. People will make their minds up on the basis of policy."

An analysis of today's Guardian/ICM poll shows the Lib Dems are drawing support from all directions, including from some people who did not vote at the last election.

Labour and Conservative hopes that the Lib Dem breakthrough is illusory are not borne out by the data, which suggests existing party supporters are also moving strongly to Clegg's party.

The Lib Dems are picking up most votes from Labour, which is down eight points on its 2005 election performance which had been the lowest in recent times for a governing party.

In today's poll, the Lib Dems have gained twice as much support from Labour as from the Conservatives.

Meanwhile, Labour is losing support in all directions, including to the Conservatives and other smaller parties. The Tories are losing almost no support to Labour and are gaining almost as many former Labour votes as they are losing former Tories to the Lib Dems.

Clegg still lags in third place as the man voters see as the most competent prime minister, on 21% against 32% for Cameron and 30% for Brown.

But he trumps his rivals by being seen as a man who is more about substance than spin. Cameron fails on this, with 44% thinking him insubstantial against 29% who say that of Brown and only 19% of Clegg.

Clegg is also seen overwhelmingly as the most honest party leader. While 53% think Cameron is honest about his policies and their implications and 51% think Brown is honest, 74% say so of Clegg.

• ICM Research interviewed a random sample of 1024 adults aged 18+ by telephone on 16-18 April. Interviews were conducted across the country and the results have been weighted to the profile of all adults. ICM is a member of the British Polling Council and abides by its rules.