Iain Duncan Smith is awaiting the outcome of the vote of confidence in his leadership.

The ballot opened at 3.30pm with former chancellor Ken Clarke and former shadow agriculture minister Ann Winterton among the first to cast their votes.

Duncan Smith loyalist Angela Browning was outside the room acting as an unofficial teller counting MPs as they arrived.

Voting was swift, with two thirds of the parliamentary party entering the polling booth within the first hour.

It is believed that Adrian Flook was the last MP to vote, and counting of the votes will begin at 6.30pm.

The Conservative leader, who had earlier made a passionate plea to his colleagues, voted shortly before 4.00pm.

Emerging from the committee room, he announced: "I did vote for myself."

The embattled leader had earlier addressed a packed meeting of the party's 1922 Committee.

Aides described the 25 minute speech as "blazingly honest" and "straight from the heart".

He arrived shortly after 2.30pm, describing his walk through the assembled media as "gladiatorial". He completed his address at 2.58pm - with many of his assembled colleagues thumping the table in support.

Michael Portillo, David Davis and Michael Howard all entered the room as chief whip David Maclean looked on.

Quizzed by the press, most MPs said they had "no idea" what the outcome would be.

Said one frontbencher: "I don't know which way it will go. I have not been crunching the numbers."

Sources suggest the Duncan Smith team has secured the public support of 72 MPs, leaving the result too close to call.

Others say MPs have been reluctant to state their real intentions fearing a backlash from constituency associations.

Over 120 journalists lined the corridor to catch snippets of the Tory leader's make-or-break address.

"We must end the bitterness that has tarnished our party, not just for two years but for the last 10 years," Duncan Smith is reported to have said.

He also admitted to having "made mistakes" during his two years as leader.

Emerging from the room, one senior Conservative MP described the speech as "very moving".

Edward Leigh said it was "the best speech he has ever made".

Shadow defence secretary Bernard Jenkin added that he had delivered "a very passionate, very emotive, excellent speech".

But Jenkin, a key lieutenant of the Tory leader, later refused to be drawn on which way the vote would go.

David Davis, tipped to enter the race as early as tonight, said: "He made a very good speech. Who knows? You never know."

Another frontbencher suggested he would be "okay", having answered three critical questions about his leadership.

A former Cabinet minister said the Tory leader had "won himself quite a lot of credit" over the last three weeks. But he questioned whether the speech would be enough to address the lingering doubts about Duncan Smith's leadership.

And a member of the Shadow Cabinet said Duncan Smith "deserves to win".

Immediately after his 25-minute address, loyalists took to the media to praise their leader.

Ann Widdecombe said he had met the demands of the occasion.

However others predicted the public praise of Tory MPs masked a private intention to stab Duncan Smith in the back in the secret ballot.

Tory chairman Theresa May claimed rank-and-file members were calling Central Office saying they "just don't understand" why MPs had triggered the vote.

"They are saying 'We elected Iain Duncan Smith as leader'. They feel he was elected to take the party through to the next general election. They want him to be given the opportunity to do that," she said.

The Tory leader's day began early with a round of appearances on all the main TV and radio news programmes.

As he toured the studios Duncan Smith warned his party not to plunge itself into a fresh leadership election.

Interviewed on the Today programme, he vowed to remain in office even if he wins the no confidence vote by a slender margin.

"As far as I am concerned, the rules are very clear. This is the one way of settling this for once and for all. It is as simple as that," he said.

And as the party hierarchy moved to shore up support, the Conservative Board endorsed Duncan Smith's leadership.

A statement said that members of the board "would like to be totally associated" with an earlier endorsement of the leader backed by Michael Howard, Michael Ancram, Oliver Letwin and Theresa May.

Prior to the vote Duncan Smith went head-to-head with Tony Blair at the weekly session of prime minister's questions.

The Tory leader entered the Commons chamber to loud cheers from the Labour benches.

With his first three questions Duncan Smith put in a robust performance, landing punches on the government for its failure to cut crime.

But he was mocked by Labour MP Dennis Skinner, who said the government should make "backstabbing a criminal offence".

Blair conceded that the crime "had gone up over recent years" but suggested the government would be powerless to act ahead of tonight's vote.

Flanked by shadow ministers including David Davis and Theresa May, Duncan Smith was subjected to chants of "bye bye" from the Labour benches.

John Prescott led Labour MPs in waving to Duncan Smith has he left the Commons chamber to prepare for the most critical speech of his leadership.