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

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.

And Tory former chancellor Kenneth Clarke said Mr Duncan Smith should not be giving "unreserved support for whatever comes out of Washington".

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.

Polling rivals

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.

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".

America's poodle

Kenneth Clarke told BBC News 24's One-to-One he did not think the Tories should be demonstrating "that we are more enthusiastically behind George W Bush even than Mr Blair is".

"I think plenty of Conservatives think the foreign policy is not to be America's poodle."

Mr Clarke said the Conservative party was "not going badly, it just isn't going". "We have got to see some product," he said, adding that the party was in need of "some policies" and "new ideas".

He attacked Mr Duncan Smith's concentration on the vulnerable and stressed: "I don't think it is remotely clear what a Tory government would do."

"The key thing about this conference is the party's got to do something to make itself seem relevant, to get people interested in it, to make some sort of impact," he said.

"It's no good us going on about the vulnerable - we need flesh on the bones.

"What exactly would we do, not only about the vulnerable and the problem of the urban poor, which is very serious, but what about the health service ... transport, foreign policy and what about the economy?"

But Mr Clarke said the Tory leader had achieved something in getting the party to "shut up" about Europe.

'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."

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.