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Nikki Da Costa was head of legislative affairs at Number 10 until late last year.

She told Adrian Chiles on BBC Radio 5 Live: "At the end [of Theresa May's speech], I was in tears.

"I think the speech was incredibly dignified...

"She was able to touch on what motivates her and what's important to her, and the genuineness of her love for her country.

"Believe me, with the hours, the intensity and sheer pressure of the job, you've got to have that love for your country to take you through it.

"It's a double bind particularly for female prime ministers... are you emoting, are you emoting too much? What does that mean about you? But I am glad that people were able to see in the genuine emotion in her speech, more of her character, more of the person that I knew."

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"Her best wasn't good enough," says Daily Mirror political editor Kevin Maguire.

"Europe, as an issue, has now destroyed the last four Conservative prime ministers," he tells the BBC.

He says Theresa May was "dealt a difficult hand and played it badly".

Irish premier Leo Varadkar has released a statement, saying he got to know Mrs May "very well" over the last two years.

"She is principled, honourable and deeply passionate about doing her best for her country, and her party," he said.

"Politicians throughout the EU have admired her tenacity, her courage and her determination during what has been a difficult and challenging time.

"Theresa May strove to chart a new future for the United Kingdom. I want to wish her the very best for the future."

With this morning's confirmation that Mrs May will remain PM until her successor is chosen, it's now clear that she will serve longer than Gordon Brown did.

She will officially surpass Mr Brown's length of term next Wednesday, when she hits two years, 320 days in office.

When she eventually leaves Number 10 in the summer, however, her premiership will still be one of the shortest.

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Theresa May was elected Conservative MP for Maidenhead in May 1997.

During her time in opposition she held a number of positions including shadow secretary for education.

Under David Cameron she served as home secretary from May 2010 to July 2016 - the longest period of time any politician has held that position.

She became prime minister in July 2016, succeeding Mr Cameron, who resigned in the wake of the EU referendum.

Scottish Secretary David Mundell said he was "very sorry it has come to this".

In a statement he said: "Nobody could have worked harder, or shown a greater sense of public duty, in delivering the result of the EU referendum than Theresa May.

"She has my utmost respect for those endeavours, in the most challenging of circumstances, as well as her unswerving commitment to the union.

"As Mrs May herself acknowledges, she has, however unfairly, become an impediment to the resolution of Brexit, and was no longer being given a hearing by Parliament."

Environment Secretary Michael Gove calls Mrs May's speech "moving".

While Steve Baker, who strongly opposed Mrs May's Brexit deal, called it "very dignified".

And Dominic Raab, a candidate to replace Mrs May as party leader, called her a "patriot" and "loyal Conservative".

Former Commons leader Andrea Leadsom, Work and Pensions Secretary Amber Rudd, and Chief Whip Julian Smith have reacted to Mrs May's announcement.

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Another Conservative MP, Pauline Latham, praises Theresa May as "very sincere and determined".

"But she didn't manage it," she adds.

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Conservative Peter Bone says the next leader has to be someone "who believes in Leave" and someone not associated with the Brexit deal.

"Either Esther McVey, Dominic Raab, David Davis or Boris Johnson.

"We also need someone who can walk on the world stage straight away and who can win the next election.

"I think the right person is Boris Johnson."

International Trade Secretary Liam Fox and Foreign Secretary Jeremy Hunt have been quick to post their tributes to Mrs May...

Welsh Conservative leader Paul Davies AM thanked Mrs May for her dedication and commitment to Wales over her many years in politics.

"It is a tremendous achievement to be the country's second female prime minister, who stepped into the job at a difficult time," he said.

"The party must now come together and deliver the Brexit that people voted for."

Conservative MP Peter Bone says the prime minister has "made the right decision".

"She could have been a national hero if she had kept to her word that we would leave the UK on 29 March," he tells the BBC.

"From that point on it was inevitable that she would not serve for much longer."

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Reacting to the PM's announcement, BBC Assistant Political Editor Norman Smith says he thought it was a "dignified but quite defensive" statement.

He adds that it contained a "clear message" to her successor that "compromise is not a dirty word".