Mr Nimmo, 68, has not regained consciousness since the incident on Wednesday night.

He was listed as "critical" in intensive care at the Atkinson Morley Hospital in Wimbledon, south west London, which specialises in head injuries.

The actor was transferred there from the Chelsea and Westminster Hospital.

Doctors are believed to have discovered a brain injury. The next 48 hours are said to be crucial to his recovery.

In a statement, his agent Barry Purnett said Mr Nimmo's wife of 43 years, Patricia and one of his sons were at his bedside. The actor has two sons Tim and Piers, and a daughter, Amanda.

Mr Nimmo was transferred from Chelsea and Westminster Hospital

Mr Nimmo overcame a stutter to launch a comic acting career in the 1960s. He was most in demand to play vicars or aristocrats.

He is still a regular panellist on the BBC Radio 4 show Just A Minute, though he originally joined the programme to help him get over his speech problems.

He now works as a theatre impresario and is a popular after-dinner speaker, spending much of his time abroad.

Mr Nimmo is also known for backing unconventional health tonics, such as bats' and lizards' blood - in line with Far Eastern practices. On Radio 4's Desert Island Discs he picked a sack of garlic as his luxury because of its health properties.

He had a quadruple heart bypass 11 years ago. Two years ago he crashed his Rolls-Royce into three cars and a greenhouse in Buckinghamshire after the accelerator pedal became stuck.