Suella Braverman has defected to Reform UK, the third sitting Conservative MP to join Nigel Farage’s party in little more than a week.

Arriving as a surprise speaker at a Reform event in London, Braverman, who has been an MP since 2015 and served as home secretary under Liz Truss and Rishi Sunak, said: “Today am announcing that I resign the Conservative whip. I have resigned the Conservative whip and my party membership of 30 years because I believe with my heart and soul that a better future is possible for us.

“I feel like I have come home.”

The Conservative party’s response was scathing. A spokesperson said: “It was only a matter of when, not if, Suella would defect.”

Without providing any evidence for Braverman suffering from mental ill-health, they said: “The Conservatives did all we could to look after Suella’s mental health, but she was clearly very unhappy. She says she feels that she has ‘come home’, which will come as a surprise to the people who chose not to elect a Reform MP in her constituency in 2024.

“There are some people who are MPs because they care about their communities and want to deliver a better country. There are others who do it for their personal ambition.”

Nigel Farage, who unveiled Braverman in front of a rally of veterans, told reporters afterwards that they had been speaking for at least a year.

Saying she understood her defection would be difficult to take for some in her constituency, Braverman, who also served as attorney general, said she had not taken the decision lightly.

The Fareham and Waterlooville MP said: “Britain is indeed broken. She is suffering. She is not well. Immigration is out of control. Our public services are on their knees. People don’t feel safe. Our youngsters are leaving the country for better futures elsewhere.

“We can’t even defend ourselves, and our nation stands weak and humiliated on the world stage. So we stand at a crossroads. We can either continue down this route of managed decline to weakness and surrender. Or we can fix our country, reclaim our power, rediscover our strength.”

Such was the secrecy surrounding Braverman’s defection that Reform staff at the event were taken by surprise.

Braverman, whose husband had previously joined Reform, was long seen as a possible defector. She follows Robert Jenrick, who was shadow justice secretary and defected on 15 January, and Andrew Rosindell, who made the move a few days later. Her move means Reform now has eight MPs.

Farage introduced her without warning at the event, which was primarily about launching a group called Veterans for Reform. After joking about how Jenrick initially did not appear when called at his defection press conference, he then introduced Braverman.

The defection is likely to worry some in Reform. Braverman was sacked as home secretary by Truss and Sunak, the first time for sending official documents from a personal email to another Tory MP, and the second after being accused of fuelling far-right violence with her rhetoric.

Reform figures have briefed in the past that neither Braverman nor Truss would not be welcome in the party.

Braverman told the event she would be setting out her reasons for leaving the Tories later in the day, but used her speech to lambast previous Conservative governments.

She revisited her sacking by Sunak, saying she had gone to him when she was home secretary to express he concerns about the impact of the European convention on human rights (ECHR) on Britain’s ability to enforce immigration policies.

“First he ignored me and then he sacked me,” she said.

Farage is coming under growing pressure over claims that Reform is turning into another Conservative party, and he was reluctant to be drawn on whether Truss could be next to join his party.

Reform’s press team told the Guardian last week that Truss would never join the party. Asked again on Monday about closing the door to her entry, Farage replied: “I didn’t say that. I said it was unlikely.”

There are now more former members of the Truss front bench in his party than in Kemi Badenoch’s shadow cabinet.

Asked about his previous description about Braverman as “absolutely pathetic” on immigration, he responded: “She was utterly useless, as they all were. They all were utterly useless because they were stuck within the ECHR. So she found herself in this bizarre position.

“I only want people who are prepared to say, do you know what, we got this horrific view wrong. There’s a lot we can learn from it. I’m going to come into Reform and put our shoulder on the wheel. And once I’m convinced they’ve passed those criteria, they can come, simple as that.”

Labour seized on the defection. The party’s chair, Anna Turley, said: “Nigel Farage is stuffing his party full of the failed Tories responsible for the chaos and decline that held Britain back for 14 years.

“Suella Braverman helped botch Brexit and got sacked as home secretary. Her defection shows Farage is willing to accept the very worst of the Conservative party and exposes his complete lack of judgment.

The Liberal Democrat’s deputy leader, Daisy Cooper, said: “Farage has recruited yet another Conservative minister with selective amnesia, one who complains about broken Britain while conveniently forgetting they helped break it.

“Suella Braverman was so bad she was forced to resign from Liz Truss’ cabinet and got sacked by Rishi Sunak.”