London's police watchdog will vote on Sir Ian Blair

It follows the conviction of the force for breaching health and safety laws over the shooting of Jean Charles de Menezes in 2005.

The meeting of the Metropolitan Police Authority is the most important since its inception in 2000.

If Sir Ian loses the vote, pressure will mount on Home Secretary Jacqui Smith to withdraw her support for him.

The Metropolitan Police was found by a jury at the Old Bailey to have endangered the public as officers hunted suicide bombers in the aftermath of the failed 21 July attacks.

'Untenable' position

Sir Ian has been given public support by Prime Minister Gordon Brown and London Mayor Ken Livingstone.

Q&A: Met chief vote

But the shadow home secretary, David Davis, has led a chorus of critics who have repeatedly said his position is untenable in the wake of the Stockwell failures.

Earlier this month a majority of members of the London Assembly agreed a vote of no confidence in Sir Ian.

His opponents were handed even more critical material by the belated publication of an independent inquiry into the death of Mr de Menezes.

The meeting of the police authority, at its base near New Scotland Yard, may yet be held in secret.

But members of Mr de Menezes' family, who will attend, fiercely oppose any attempt to hold the meeting behind closed doors.