Voting has closed today in the contest to become Britain's next prime minister, with Boris Johnson expected to be confirmed as the winner charged with delivering Brexit.

After a month-long contest between former London mayor Johnson and Foreign Secretary Jeremy Hunt, the postal votes of up to 160,000 grassroots Conservatives will decide the governing party's next leader.

The voting window closed at 5pm.

The result will be announced tomorrow, with the winner immediately becoming the new Conservative leader, the victor taking office as prime minister on Wednesday.

Both candidates have had a rocky end to the campaign.

Chancellor Philip Hammond announced yesterday that he would make a point of resigning before Mr Johnson became prime minister, saying he could never agree to his Brexit strategy.

The final posted votes will be delivered to Conservative headquarters in London this morning. Any last remaining votes will have to be delivered by hand or courier.

An online poll of 1,199 members conducted on Friday and Saturday by the Conservative Home website put Mr Johnson on 73%.

Bookmakers give Mr Hunt around a one in 15 chance of winning.

The Conservatives command a razor-thin majority in parliament's lower House of Commons and Mr Johnson's opponents -- both within and outside the party -- are keen to scupper his leadership.

Mr Johnson has vowed to take Britain out of the European Union on 31 October, with or without a divorce deal.

Opponents of Brexit, and especially of a no-deal departure, are plotting moves against Mr Johnson.

Some Conservatives, Mr Hammond included, have hinted they are prepared to bring down their own government rather than accept leaving the EU without an agreement.

Mr Hammond has no illusions of remaining in post under a Johnson premiership, and has said he will resign before being moved on in a reshuffle.

"I cannot accept the idea of leaving with no deal on 31 October," Mr Hammond said.

Justice Secretary David Gauke also said he would quit the government if Mr Johnson became prime minister.

Meanwhile, Foreign Office Minister Alan Duncan today resigned from his post ahead of the leadership announcement. Mr Duncan has been a staunch critic of Mr Johnson, once describing him as a "circus act".

The Sunday Times newspaper reported that up to six europhile Conservative MPs were considering defecting to the centrist, pro-EU Liberal Democrats should Mr Johnson win - leaving him without a Commons majority.