London: The voting closes on Monday 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 Boris Johnson and foreign secretary Jeremy Hunt, the postal votes of up to 1,60,000 grassroots Conservatives will decide the governing party's next leader. The voting window slams shut at 5pm (1600 GMT).

The result will be announced on Tuesday, 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. Finance minister Philip Hammond announced on Sunday that he would make a point of resigning before Johnson became Prime Minister, saying he could never agree to his Brexit strategy.

Meanwhile Hunt has had to deal with the fallout of Iran's seizure of a British-flagged tanker in the Gulf.

In one of her final acts as Prime Minister, Theresa May will chair a meeting of Britain's COBR emergencies committee at around 10:30am(0930 GMT).

"As well as receiving the latest updates from ministers and officials, the COBR meeting will discuss the maintenance of the security of shipping in the Persian Gulf," a Downing Street spokeswoman said.

Hunt said Parliament would be updated on Monday with respect to the situation in the Gulf.

The final posted votes will be delivered to Conservative headquarters in London on Monday 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 Johnson on 73%.

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

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

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

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

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