Lewis has been working in TV news, both at ITN and the BBC, for 32 years. With him as presenter, the Six O'Clock News has become television's most watched news programme.

But it was revealed last December that Lewis had not been selected to front the revamped Six O'Clock News. The bulletin is to be relaunched on 10 May with Huw Edwards as main presenter of the programme.

Lewis had been asked to take a prominent role with the global BBC World channel, but has instead decided to leave the corporation's news service.

He said: "I have been privileged to be involved in the coverage of many major news stories. The Six O'Clock News has a loyal army of viewers. I shall of course, miss them as well as the cut and thrust of the daily news challenge.

"But I have no doubt that Huw Edwards will bring to the programme a distinctive approach which they will like. I wish him every success for the new-look Six O'Clock News."

'A delight to work with'

Huw Edwards: Will host the new bulletin from 10 May

BBC News chief executive Tony Hall paid tribute to Lewis' career, saying: "It would be difficult to overstate the contribution you have made to the development of the Six O'Clock News as the nation's most watched news programme.

"You have also been a delight to work with and your humanity has shone through as you have brought the nation news of some of the gravest and most momentous events of our times."

Huw Edwards said: "I did my first ever Six with Martyn five years ago. He was as helpful and as generous with his advice as he could possibly be. He is going to be a hard act to follow."

Lewis' current Six O'Clock News colleague Moira Stuart is to stay on screen, joining the BBC's Breakfast News team.

His departure came as it was announced that former Grandstand presenter Helen Rollason is to front a new sports bulletin as part of the new programme.

Rollason won widespread admiration last year when she spoke of her battle against liver and colon cancer in a television documentary.

Rollason joins team

Helen Rollason: Joins the new bulletin in May

In March 1998 doctors told her she only had six months left to live, but on Sunday she was among the team which started 31,000 runners on the London Marathon.

Now she will present an in-depth preview of the weekend's sporting action every Friday from 14 May.

The programme's new editor, Mark Popescu - who joined the BBC from ITN's now-defunct News At Ten - said: "Sport has been fairly poorly covered by news, so we're going to do more than we have in the past.

"The Friday sports bulletin will help to underline our commitment to sports coverage ahead of Wimbledon, the Cricket World Cup and the climax to the football season. Helen knows her sport and she engages all viewers - not just sports fanatics."

Rehearsals are under way for the new bulletin, which will have a "warmer" set and two new presenters. Huw Edwards will host the bulletins on Monday to Thursday, with Fiona Bruce presenting on Friday as well as fronting location reports.