Sir Liam took up the role in 1998 and is the 15th person to hold the position since it was established in 1855. He is longest serving CMO of modern times.

Sir Liam had originally envisaged leaving his post when he turned 60 in mid-2009. He agreed to stay in his role to supervise the response to the swine flu pandemic. He has stated that, if the pandemic should unexpectedly worsen, he will extend his tenure beyond May 2010.

In his resignation letter Sir Liam said he had been immensely privileged to serve in the post.

‘I have been pleased to see many of my policy recommendations - stem cell research, smoke-free public places, reforms to the GMC, changes to consent for organ and tissue retention and the creation of the Health Protection Agency - carried forward into legislation,' he said.

‘I have been pleased too, that reforms I proposed to improve quality and safety of NHS care - clinical governance, a patient safety programme, procedures to identify, and prevent harm from, poor clinical practice - are fully embedded in the service and have been also adopted in many other parts of the world.'

Prime minister Gordon Brown said Sir Liam had made an ‘extraordinary contribution' to the nation's health.

'His leadership and action in these areas and others will have saved many, many lives,' he said. ‘I and the whole country are extremely grateful for all he has done and wish him all the best for the future.'

Health secretary Andy Burnham said Sir Liam had fulfilled his duties with 'great distinction, wisdom and good humour'. 'He has brought courage and foresight to the role of improving the nation's health,' he added.

'His bold and once-controversial proposal to turn public places smoke-free shows the difference he has made.'