Iain Duncan Smith has been warned he must stamp his authority on the Conservative Party at its conference in Bournemouth next week.

The Spectator magazine, edited by Tory MP Boris Johnson, says in its cover story that the Tory leader's time is "running out" unless he delivers a convincing performance at Bournemouth.

Sir Malcolm is calling for a more critical stance

And former Defence Secretary Sir Malcolm Rifkind said Mr Duncan Smith was damaging the party's credibility with his "unquestioning" support for Labour's Iraq policy.

Meanwhile, in a poll for the Daily Telegraph, Mr Duncan Smith trailed behind Liberal Democrat leader Charles Kennedy as the voters' choice for an alternative prime minister.

The NOP poll suggested just 11% of voters believe Mr Duncan Smith would make the best prime minister compared to 19% for Mr Kennedy.

Tony Blair was the most popular choice with 36%.

Overall the NOP poll suggested Labour was still well ahead with 41%, a 13 point advantage over the Tories on 28%, while the Lib Dems had 21%.

Tory sources have suggested the poll is not in line with recent research, which suggests the party is closing the gap on Labour.

An ICM poll for The Guardian last month suggested Labour's lead over the Tories had dropped sharply to just 5%.

'Intelligent criticism'

In an article for The Spectator, Sir Malcolm Rifkind says the Tories are failing in their constitutional duty on Iraq.

I am not sure (Iain's) team really understands what the modernisation agenda is about

Steve Norris

"An opposition that is seen to be giving uncritical and unqualified endorsement to the government on such a fundamental issue is unlikely to enhance its credibility as an alternative and preferable government," Sir Malcolm writes.

He said the "whole purpose of Her Majesty's Opposition" was to "hold the executive to account".

Sir Malcolm, who supported Kenneth Clarke in last year's Tory leadership contest, argues that "intelligent criticism" by the opposition does not have to mean a "descent into crude partisanship".

The time for unequivocal support from opposition parties was when the shooting started, Sir Malcolm argues.

Leadership challenge?

On its cover, The Spectator warns "Your time is running out, Iain".

Inside, the magazine's political editor, Peter Oborne, warns the mood among the Tory rank and file on the eve of their conference was "febrile".

There was even talk of possible leadership challenges if Mr Duncan Smith fails to convince in Bournemouth, Mr Oborne claims.

He said the party was "facing the worst crisis in its history", following the removal of David Davis as party chairman, the departure of strategy director Dominic Cummings and internal feuds over its future direction.

'Missing the point'

Meanwhile, Conservative former minister Steve Norris has attacked Mr Duncan Smith's "intellectually incoherent" attempts to modernise the party.

The former London mayoral candidate told the Guardian newspaper: "I am not sure (Iain's) team really understands what the modernisation agenda is about."

"It is not about being Blairite.

"It is about getting to where most of the electorate has been for a long time on issues like sexual orientation."

He said the Tory leader was "missing the point" by attempting to cast the Tories as the party of the "poor, the dispossessed and the marginalised".

Mr Norris is expected to make the case for the repeal of Section 28, which bans the promotion of homosexuality by local authorities, at a Bournemouth fringe meeting.

More diary troubles?

In the light of this week's revelations about John Major's affair with Edwina Currie, senior Tories are also understood to be nervous about the publication of the final volume of former defence minister Alan Clark's diaries.

The Times is due to publish extracts from the book, which covers John Major's premiership and the first part of William Hague's tenure, on Saturday.

Meanwhile, Mr Duncan Smith has been faced with the defection of a backbench MP.

Andrew Hunter, MP for Basingstoke, has said he will leave Westminster at the next election to stand for the Northern Ireland Assembly as a member of the Democratic Unionist Party.