Gordon Brown’s leadership standing has now fallen below that of Iain Duncan Smith during his short-lived and unhappy period as the Conservative Party leader, according to the latest Populus poll in The Times today.

The poll underlines the seriousness of Mr Brown’s position as he faces a knife-edge vote in the House of Commons tomorrow evening on extending pre-charge detention for terrorist suspects from 28 to 42 days.

Public support for the Prime Minister and his party is plunging to new depths and ministers fear that a defeat tomorrow would leave him even more exposed. Mr Brown is making personal appeals to MPs after a fresh assessment by the Labour whips told him that the vote remained extremely tight with no guarantee of victory.

Labour’s rating has fallen by four points since last month to 25 per cent, with the Conservatives up five points to 45 per cent. This is one of Labour’s worst poll ratings and the best Tory one since 1997. The Liberal Democrats are up one point at 20 per cent.

David Cameron’s lead has increased despite the damage done to the Tory image by allegations about the party’s MEPs. The number of voters saying that the Tories are tainted by financial sleaze has jumped by ten points since February to 61 per cent.

The alarming feature for Labour is that there is no sign of any halt to the eight-month decline in its — and Mr Brown’s — rating. The position is as bad as it was for the Tories in the mid-1990s.

Moreover, Mr Brown no longer enjoys the benefit of the doubt among voters. His leadership rating (on a 0 to 10 scale of very bad to very good) has fallen for the fifth time running, down from a peak of 5.49 last July to 3.9 now. This is lower than any of the other seven leaders of the main parties since the index was introduced five years ago. The previous low of 4.00 was for Mr Duncan Smith in 2003.

Mr Brown’s rating is particularly poor among the fifth of Labour’s voters in 2005 who now say that they would back the Tories — down at just 3.55. Mr Cameron’s personal rating, at 5.25, is at the top end of his range.

The sole consolation for Mr Brown is that nearly three quarters of voters back the 42-day proposal. Forty per cent say that an increase to 42 days’ detention or higher is justified because of the seriousness of the threat, while 33 per cent say that a rise is only justified if safeguards on parliamentary monitoring are offered. In an unusual public statement last night the security service MI5 rejected suggestions that it opposed the plan and said that it recognised the challenge posed by increasingly complex cases.

The latest poll comes as ministers and Shadow ministers admit that tomorrow’s vote is likely to be decided by the votes of the nine Democratic Unionist MPs. All Cabinet leave has been cancelled with David Miliband, the Foreign Secretary, coming back early from a visit to the Middle East. MPs who are unwell are being told that they will be needed in the House.

A victory for Mr Brown will ease the immediate pressure on him, although the poll suggests that it might only be a temporary respite. The Government’s working majority is 66, meaning that if 34 Labour MPs vote against the plan, it would be defeated should all the opposition MPs in every party turn up and oppose it.

The Democratic Unionists opposed the plan to increase detention to 90 days in 2005, but they are now being wooed by both the Government and the Tories. If they abstain, it would take 43 Labour rebels to defeat Mr Brown. If they vote with the Government, the rebel number would have to soar to 52 for the critics to win.

Jeffrey Donaldson, the DUP MP for Lagan Valley, said: “We will take a collective decision but none of us has indicated our personal preference. If we were going to do a deal, we would not be broadcasting it until negotiations are complete.”

The Government has denied offering extra money for Northern Ireland in return for support from ther DUP. Among the deals mooted has been a request from the party that money from the sale of former military bases should go to the Northern Ireland executive rather than to the Ministry of Defence.

Should the Government win the debate, Trevor Phillips, chairman of the Equality and Human Rights Commission, said that his organisation would move “immediately” to test the legality of the proposal in the High Court.

— Populus interviewed a random sample of 1,508 adults across the country aged over 18 between June 6 and 8. The results have been weighted to be representative. For more details go to www.populus.co.uk.

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