IAIN DUNCAN SMITH was facing sustained pressure over his leadership last night as plotters were accused of trying to destroy him and a new poll showed that most Tory voters believe that another leader would help their chances.

The Tory conference opens in Blackpool today with Mr Duncan Smith under siege for the second year in succession.

In an eve-of-conference interview, he issued a direct and general threat to sue anyone who made allegations referred to in yesterday’s newspapers about the management of his office.

The claims about payments to his wife were reported by senior leadership figures to be the work of a clique of disaffected modernisers who were determined to bring Mr Duncan Smith down “by hook or by crook”.

Today’s Populus poll in The Times shows that Conservative supporters are keener than other voters to replace Mr Duncan Smith and that Labour remains five points ahead of the Tories — 36 per cent to 31 per cent — despite all Tony Blair’s recent problems. Kenneth Clarke is the favoured candidate of the public generally and Tory supporters, although Michael Howard is also popular among Tory voters.

Mr Duncan Smith’s rating as leader has slipped for the second month running and remains the lowest of the three party leaders among voters generally and among his own party’s supporters.

Francis Maude, the former Shadow Chancellor, said publicly that the leadership was an issue. Privately Tory MPs were telling political journalists that Mr Duncan Smith could face a challenge before Christmas.

In spite of the grim polling news Mr Duncan Smith is, surprisingly, more upbeat about his party’s position than he has been since becoming leader. He has told friends that he regards this conference as the pre-election springboard for announcing policies and campaigning on them. He will use his speech on Thursday to launch his strongest personal attack on the Prime Minister, believing the public now has greater doubts about Mr Blair than ever before.

Friends of the Tory leader say that he is ready to pursue his legal fight to protect his name to the end. He has said privately that the allegations are malicious and utterly false and that he will not allow his wife to be dragged into what he believes are the politics of the gutter.

Mr Duncan Smith and his team will try to rally the party by unveiling parts of the platform on which they will fight the next election.

Liam Fox, the Shadow Health Secretary, will today pledge Bank of England-style independence for a new board to run the funding of the health service free from political interference, The Times has learnt.

Yesterday the Conservatives promised to restore the link between the basic state pension and earnings — abolished in 1980 — to lift one million pensioners out of means-testing and raise their income by £11 above inflation for a couple by the end of a first Tory term.

And Mr Duncan Smith set out to draw a contrast between his “quiet man” leadership and that of Mr Blair.

He said: “There are two types of leadership. There is the Blair leadership, which is full of promise and lots of glitz and glamour and goes around shouting the odds and then the delivery is non-existent, he fails and then he lies about it.

“Then there is the other type of leader who puts in place the firmest policies and the absolute strategy they will stick to. I have stuck to that.

“That sort of leader then takes that to people and says ‘This is me, this is who we are and this is what we stand for’. I can tell you that’s what I’m doing.”

Mr Duncan Smith also shrugged aside talk of division between himself and Mr Howard over tax cuts. As the Shadow Chancellor continued to voice caution, Mr Duncan Smith said the Conservatives would bring forward their plans after next year’s Budget and spending review.

Mr Duncan Smith’s aides said that his lawyers were studying yesterday’s reports relating to his personal affairs.

The Times is aware that BBC’s Newsnight had been planning to run a story last week about the issues referred to in the reports, but it was blocked by senior executives.

Mr Duncan Smith, interviewed on BBC’s Breakfast with Frost, said that any allegations of improprieties were “false lies”. He said that if anybody made such allegations, “I will sue them”.

“I have a whole legal team who have been looking at this and I can tell you absolutely that they will find themselves with a lawsuit.”

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