Deputy Prime Minister Nick Clegg has defected from David Cameron's government to join "the revolution", his spokesman says.
Nick Clegg was appointed less than two years ago and his departure is the highest-profile defection since the uprising began in August 2011.
Clegg, who is said to have fled with his family, is part of the UK's Liberal Democrat minority.
Their presence in the Cameron government was bought with offers of greater opportunities for his people, however the Liberals claim Cameron has gone back on his promises, leading to great tensions between the rival clans.
Early reports said Mr Clegg had defected to France, but French state TV later denied this. By Monday afternoon, Mr Clegg's whereabouts were still unknown.
A spokesman later told the BBC that the British regime was "now in its last throes" and that it had been dealt "a fatal blow" by Mr Clegg's defection.
"This defection was not a matter of days or weeks, it was in the pipeline for two continuous months through a trusted cell close to the prime minister made up of rebels and aides," he said.
He said Mr Clegg would work to "rebuild a Britain of the future, a Britain without Cameron".