British Prime Minister Boris Johnson lost his working parliamentary majority ahead of a crucial Brexit vote on Tuesday after Conservative MP Phillip Lee defected to the pro-EU Liberal Democrats.
"The Liberal Democrats are delighted to announce that Bracknell MP Phillip Lee has joined the party," the Lib Dems said in a statement.
Johnson was braced for a showdown in parliament on Tuesday over his Brexit plan in a vote that could derail Britain's exit from the European Union next month.
Members of Johnson's own Tory party, including Lee, have worked with opposition lawmakers to draft a bill to force him to delay Brexit if he cannot agree divorce terms with the EU in time.
Johnson has warned he will prevent any Tory MPs who support the bill from standing for the party in any upcoming election, which is looking increasingly likely.
"This Conservative government is aggressively pursuing a damaging Brexit in unprincipled ways," said Lee.
"It is using political manipulation, bullying and lies. And it is doing these things in a deliberate and considered way," he added.
Liberal Democrat leader Jo Swinson said she was "delighted" by the defection.
"He shares our commitment to prevent a disastrous No Deal Brexit, and to stop Brexit altogether," she said.