A councillor's death meant the Tories were in a minority of one at the council meeting.
They walked out after defeat of their call to adjourn out of respect for Alan Kemp who died last week, leaving Labour to elect Paul Daisley as the authority's new leader.
Tories have ruled on the mayor's casting vote since the last elections in 1994, when they gained seats in the borough against the national tide. Mr Kemp's death has left them with 32 out of 66 seats, with Labour on 28 and Liberal Democrats on five. Mr Kemp's Preston seat in Wembley has been traditionally safe for Conservatives and they are likely to hold it in the by-election. However, poll rules mean the contest cannot be held until the end of May at the earliest.
The council holds its annual meeting on 15 May, at which other parties will have the opportunity to elect a Labour or Lib Dem mayor, thus securing the casting vote for themselves.
Brent has a reputation for controversy. This year a consultants' report attacked as "unwise and inappropriate" a decision to expedite a pounds 70,000 grant to Irish community groups. For much of the Eighties it was dominated by the far left, bringing the nickname "Barmy Brent" from opponents.Reuse content