Former Brexit Secretary Dominic Raab has joined the race for the Tory leadership.
Mr Raab vowed to fight for a fairer deal on Brexit and said that would require ‘focus, discipline and resolve’.
His announcement now puts six candidates in the running as potential Conservative party leaders.
Other confirmed candidates are Foreign Secretary Jeremy Hunt, International Development Secretary Rory Stewart, Health Secretary Matt Hancock, former Foreign Secretary Boris Johnson, and former Work and Pensions secretary Esther McVey.
“To deliver Brexit successfully will require focus, discipline and resolve,” Mr Raab wrote in an article for the Mail on Sunday.
“As a former Foreign Office lawyer and Brexit Secretary I have the experience.”
He said he would prefer to leave the EU with a deal but said the UK must “calmly demonstrate unflinching resolve to leave in October – at the latest”.
The MP for Esher and Walton said: “The country now feels stuck in the mud, humiliated by Brussels and incapable of finding a way forward.
“The Prime Minister has announced her resignation.
“It’s time for a new direction.”
Mr Raab was a prominent Brexiteer in the referendum campaign and Theresa May appointed him as her second Brexit secretary in July.
But he quit the role in November, saying he could not support her eventual deal.
Theresa May will stand down as Tory leader on June 7 after a state visit by US president Donald Trump.
She will then remain as caretaker PM until her successor is elected by the end of July, when she will visit the Queen at Buckingham Palace and formally resign.
Power-hungry Tory leadership rivals immediately began jostling to take over, led by Boris Johnson.
Foreign Secretary Jeremy Hunt, the favourite among MPs, waited barely three hours before announcing his own run for the top job.
Mrs May stepped out through the black front door of No10 this week to admit her Brexit plans had finally hit the wall.
She said: “It is and will always remain a matter of deep regret to me that I have not been able to deliver Brexit.
“I have done everything I can to convince MPs to back that deal. Sadly, I have not been able to do so. I tried three times.
“I believe it was right to persevere, even where the odds against success seemed high.
“But it is now clear to me that it is in the best interests of the country for a new Prime Minister to lead that effort.”
Mrs May quoted her late constituent Sir Nicholas Winton, the Kindertransport hero who rescued hundreds of children from the Nazis, urging Tory MPs to compromise over Brexit.
She recalled: “He said, ‘Never forget that compromise is not a dirty word. Life depends on compromise.’ He was right’.”
Having failed to break the Commons Brexit deadlock, she finally agreed to leave.