His departure is down to disagreements over the importance attached to his character.

Lowe plays White House deputy communications director Sam Seaborn in the award-winning series.

His character will be written out of the show in the fourth series, which is yet to be filmed.

Lowe spoke out about his decision to leave on Wednesday.

We're fortunate that he's going to be with us for virtually the entire year and after that, Sam Seaborn will move on to other things

Jeff Zucker, president of NBC Entertainment

"As much as it hurts to admit it, it has been increasingly clear, for quite a while, that there was no longer a place for Sam Seaborn on The West Wing," he said.

The actor is said to be unhappy that Warner Brothers, the show's producers, will not consider raising his salary of $75,000 (�48,000) per episode.

Lowe added: "Warners had allowed me an opportunity to leave the show as I arrived - grateful for it, happy to have been on it and proud of it. We were a part of television history and I will never forget it."

In a statement, Warners described the parting as "amicable".

'Heartbroken'

Martin Sheen, who plays President Josiah Bartlet in the West Wing, recently saw his salary tripled to $300,000 (�191,000), according to trade paper Variety.

Other cast members - Allison Janney, Richard Schiff, John Spencer and Bradley Whitford - also managed to increase their pay after joining forces to pressurise Warners.

They however reportedly all still remained on less per show than Lowe.

Lowe's career has been revitalised by the West Wing

Sheen described the cast as "heartbroken" at the loss of Lowe. He added: "We have become so close."

Of Lowe's reported request for more money, Sheen said: "Everyone is entitled to go for whatever they can get. I did, certainly."

Lowe's character will be written out of the series in March, said Jeff Zucker, president of NBC Entertainment, the US TV network that screens the series.

"Rob has been a huge and great part of the programme," Zucker said. "We're fortunate that he's going to be with us for virtually the entire year and after that, Sam Seaborn will move on to other things."

Boost

Lowe began the series as the lead actor - securing both higher wages and a lead credit.

But slowly Sheen emerged as the show's kingpin character, surrounded by more of an ensemble cast.

As Sheen's airtime has grown, Lowe's has become smaller.

The West Wing is credited with reviving Lowe's career after a video emerged of him having sex with an underage girl while attending a Democratic National Convention in 1988.

Having been a successful actor during the 1980s in films such as St Elmo's Fire and the Outsiders, his career nosedived following the scandal.

Lowe was previously nominated for an Emmy and two Golden Globes for his portrayal of Seaborn.

But he was the only major cast member who did not get a nomination for this year's Emmys.

In the UK, Channel 4 is currently screening the second series. The third series is due to begin on E4 in August.