Trainspotting star Robert Carlyle has been spotted alongside author Irvine Welsh at the secret filming base where they are shooting the long-awaited film sequel.

Carlyle, who is reprising his role as the psychotic Begbie, was spotted in West Lothian on Tuesday as he met with film crew and the original book's author Irvine Welsh.

Wrapping up against the bleak weather Carlyle made a cosy display in a navy military style coat, denim shirt and t-shirt as he shook hands with a member of the film crew.

The star was snapped outside the Pyramids Business Park, Bathgate, where indoor scenes for the long-awaited follow-up are to be filmed in a converted sound stage.

Read more: Irvine Welsh announces Trainspotting sequel will begin filming in May

An excited onlooker said: "I saw him just as he was leaving the building. He said goodbye to staff before getting picked up in a car.

"I can't believe filming is finally happening. Fans have waited a long time for the sequel."

Speaking alongside author Irvine Welsh in Edinburgh last week, Scots actor Carlyle said the new script was "absolutely fantastic" and better than the first film's.

The new film - based on the sequel to Trainspotting, Porno - is being adapted by screenwriter John Hodge for the big screen and will also be shot on location in both Edinburgh and Glasgow, just like the original, which was released 20 years ago.

Carlyle said: "We start shooting, I understand, in the middle of May and I think it's going to be pretty much 50-50 between Edinburgh and Glasgow."

Trainspotting author Irvine Welsh was also spotted smiling outside the filming site. Welsh is set to reprise his cameo role as drug dealer Mikey Forrester in the sequel.

Writing on social media, the author said: "Great to visit the TS2 set and see the place teeming with life as I get fitted up for Mikey Forrester. So many old faces to catch up with."

Read more: Trainspotting 2 to be filmed in Glasgow, says author Irvine Welsh

Ewan McGregor, Ewen Bremner, Johnny Lee Miller and director Danny Boyle are all officially on board for the follow-up to the 1996 classic.

Carlyle revealed some clues to the plot at the Edinburgh event last week.

He said: "What I will say is that all the characters are exactly where you would want them to be.

"The strength of this new script is the fact that the narrative is a bit stronger than it was in the original.

"In the first one, when you think about it, it's hard to remember what they did other than get together, the drug deal at the end, and then it's over.

"This has maybe got a wee bit more to it in terms of what they've been up to through these years.

"You learn a lot about Renton, Sick Boy and indeed Begbie and where their heads are."

The sequel will be loosely based on Welsh's novel Porno, which is set ten years after Trainspotting and sees the characters cross paths again, but with the pornography business rather than heroin use as the backdrop.

Read more: Danny Boyle confirms Trainspotting 2 filming is happening NOW

Jon Hodge, who wrote the original screenplay, has also adapted the new script.

Carlyle said: "I think it's a wonderful script.

"John Hodge has done a wonderful job, as he did with the original.

"Fingers crossed people will take it on board and they will enjoy it."