A LEGENDARY British actor has signed up to appear in EastEnders for a high profile guest role.

The BBC soap has just celebrated its 40th anniversary with a dramatic live episode and a full year of comebacks of stars from its history.

But now The Sun can exclusively reveal that Trainspotting actor Keith Allen is joining the soap.

The father of popstar Lily Allen and Game of Thrones star Alfie Allen will play a mysterious new character called Gaz in a special episode surrounding Phil Mitchell and his mental health battle.

A source said: “Keith Allen is a massive signing for EastEnders.

“He’s known to millions and it’s hard to believe he’s not been in the show before – he’s perfect for it.

“He’s going to appear for a short stint in a hard-hitting storyline. It’s a huge coup for the show.”

EastEnders has worked closely with The Samaritans, Rethink Mental Illness, the Campaign Against Living Miserably (CALM), and Mind to ensure Phil’s story is portrayed as accurately and as sensitively as possible.

Set across four weeks, the episode will focus on Phil’s journey in the mental health unit from the moment he starts treatment, as his character prepares to face his inner trauma and understand what may have contributed to his depression diagnosis and his symptoms of psychosis.

And now Keith has revealed he is overjoyed at finally landing a role on the soap.

He said: “I’ve written a number one hit single, I’ve presented Top of the Pops, I’ve played the lead in the West End and I was at Craven Cottage when we beat Juventus 4-1…Could it get better than that?

“Well, yes, I’ve just guested in an episode of EastEnders…What a joy! And what an honour to be a part of Steve McFadden’s incredibly moving story line.

“I may be a resting actor but I now rest in peace.”

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EastEnders boss Chris Clenshaw added: “I’m delighted to welcome the incredibly talented Keith Allen to the cast of EastEnders as he takes on the guest role of Gaz in a special episode focusing on Phil’s mental health.

“As Phil struggles to begin treatment, we explore how his relationship with fellow patient Gaz, helps Phil to take the first step in his road to recovery.

“Keith and Steve’s performances are both phenomenal, and thoughtfully and sensitively portray the complex realities of mental health recovery and the impact of hypermasculinity.”