Former London Mayor Boris Johnson, a leading Brexit campaigner who had been considered one of the favourites to replace David Cameron as British Prime Minister, said on Thursday he would not be standing. Cameron announced he would resign last week after Britons voted to leave the European Union and Johnson had been seen as the leading candidate from the Leave camp to succeed him. "Let us seize this chance and make this our moment to stand tall in the world. That is the agenda for the next prime minister of this country," Johnson told reporters.

Gove and May throw their hats in

"But I must tell you, my friends ... that having consulted colleagues and in view of the circumstances in parliament, I have concluded that person cannot be me." Justice Secretary Michael Gove, another of the main figures in the Leave campaign who had previously said he would back Johnson, had earlier announced he would stand to be leader of the Conservative Party, saying he did not think Johnson could provide leadership.

Britain's powerful Home Secretary Theresa May today launched her bid to succeed Prime Minister David Cameron, saying the country needs strong proven leadership to negotiate the best possible terms for the UK leaving the EU.

"My pitch is very simple. I'm Theresa May and I think I'm the best person to be prime minister of this country," May, who has been the UK's longest serving home secretary, said at a press conference.

She underlined that "Brexit means Brexit" as she announced that she will be running for party leadership contest, which is set to conclude by September 9.

"Our country needs strong proven leadership to negotiate the best possible terms for the UK leaving the EU. Brexit means Brexit. The campaign was fought, the vote was held, turnout was high and the public has given its verdict," the 59-year-old Conservative party leader said, adding, "We need leadership that can unite our party and our country".

Cameron had announced that he would be stepping down after the referendum results in favour of Britain's exit from the European Union (EU) last week.

May said that among her first jobs as the Prime Minister would be to create a new government department responsible for negotiating Britain's exit from the EU, headed by an MP who campaigned for Britain to leave the EU.

May, considered as the most powerful Conservative woman since Margaret Thatcher, spoke about invoking Article 50, the formal procedure for leaving the EU, which Cameron has left for his successor to do.

"Article 50 should not be invoked before the end of the year," she said.

She argued that under her leadership the Conservative Party will come back together, not just for Remain or Brexit, but for the whole country.

"I know I'm not a showy politician...I don't go drinking in Parliament's bars. I don't wear my heart on my sleeve, I just get on with the job in front of me and you can judge me by my record," May said.

Calling for an "open contest", she added: "Whether it's a woman or a man it's about the qualities of the people doing the job."

Seen as a tough politician for her firm stance on complex issues like immigration, May has strong support within the Conservative party.

As the leadership contest continues to heat up before the Thursday night deadline for nominations, she is expected to go head to head with former London mayor and Leave campaigner Boris Johnson.

Justice secretary Michael Gove, who was expected to back the leadership bid of fellow Brexit campaigner Johnson, has thrown his own hat in the ring instead.

"I wanted to help build a team behind Boris Johnson so that a politician who argued for leaving the European Union could lead us to a better future. But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead. I have, therefore, decided to put my name forward for the leadership," Gove said in a statement.