Rebel Tory MP Phillip Lee has dramatically defected to the Liberal Democrats – depriving Boris Johnson of his Commons majority.

The pro-EU former justice minister walked across the Commons chamber, to sit with Jo Swinson’s party, meaning the prime minister now leads a minority government – the first since 1996.

“The party I joined in 1992 is not the party I am leaving today,” said Dr Lee, a supporter of a Final Say referendum on Brexit.

“This Conservative government is aggressively pursuing a damaging Brexit in unprincipled ways. It is putting lives and livelihoods at risk unnecessarily and it is wantonly endangering the integrity of the United Kingdom.

“More widely, it is undermining our country’s economy, democracy and role in the world. It is using political manipulation, bullying and lies. And it is doing these things in a deliberate and considered way.”

Under Mr Johnson, the Conservative party had become “infected with the twin diseases of populism and English nationalism”, Dr Lee warned, in a damning resignation letter.

The defection is the third to the resurgent Lib Dems in recent months, after Chuka Umunna, a former Labour MP, and Sarah Wollaston, an ex-Conservative, joined the ranks.

Protesters take to streets after Boris Johnson suspends parliament Show all 11 1 /11 Protesters take to streets after Boris Johnson suspends parliament Protesters take to streets after Boris Johnson suspends parliament Anti Brexit protesters march on Whitehall EPA Protesters take to streets after Boris Johnson suspends parliament Many of the protesters carried placards AP Protesters take to streets after Boris Johnson suspends parliament A demonstrator wears a Boris Johnson mask during a London rally AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament An anti-Brexit demonstrator wields an EU flag during a London rally. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament The protesters also marched to Trafalgar Square, where they blocked traffic AP Protesters take to streets after Boris Johnson suspends parliament John McDonnell addressed crowds in Whitehall. PA Protesters take to streets after Boris Johnson suspends parliament A small group of Brexit supporters attempted to stage a counter protest in central London. AP Protesters take to streets after Boris Johnson suspends parliament Three people were arrested in London, including a woman who sat on the road near Trafalgar Square. AP Protesters take to streets after Boris Johnson suspends parliament In Oxford, locals and students gathered outside Balliol College, Boris Johnson's alma mater, to protest. PA Protesters take to streets after Boris Johnson suspends parliament Thousands of people gather in central Glasgow to protest against the prime minister. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament Jeremy Corbyn spoke to protesters in central Glasgow. Getty

It came in the most striking fashion – as the prime minister opened his statement on the G7 summit – and, for a few seconds, most MPs didn’t realise it was happening.

Cheers then broke out, drowning out Mr Johnson as he told the Commons about global efforts to stem the fires in the Amazon rainforests.

The defection appeared to subdue Conservative MPs who failed to give their leader any rousing cheers – prompting SNP taunts that he had endured the “shortest political honeymoon in history”.

It leaves the Conservatives in an even more perilous position at Westminster, without even the tiny majority of one it had because of the deal with the Democratic Unionist Party.

Jeremy Corbyn seized on the new situation, telling the Tory leader: “This is a government with no mandate, no morals and, as of today, no majority.”

A delighted Ms Swinson said: “The Liberal Democrats are growing. Phillip follows both Chuka Umunna MP and Sarah Wollaston MP in bravely crossing the floor to join us.

“These representatives join over 30,000 new members, who have joined the Liberal Democrats since our best ever results in the European Elections in May.”

In his statement, Mr Johnson rejected claims that his Brexit strategy was floundering, telling MPs: “It is simply wrong to say that we are not making progress. There is a lot to do in the coming days but things are moving.”

The government was “utterly determined” to leave the EU on October 31 “come what may, without a deal if necessary”, he insisted.