The BBC are said to have implemented extra security measures to keep the Gavin And Stacey Christmas special plot under wraps after a huge element of the script was leaked.
The cast and crew have been filming the last ever episode of the sitcom, set to air on Christmas Day, in Barry - drawing hundreds of fans to the set.
Bosses are said to have ramped up security after a photo was taken of James Corden, who plays Smithy, holding the script, which contained the plot detail, while filming in a department store.
After the setback, producers are said to have warned the rest of the cast to be careful, especially with filming bringing so many fans to Barry.
'Everyone is excited for the return of Gavin, Stacey, Smithy and the gang so naturally interest is at an all-time high,' a source told The Sun.
'Every day fans are flocking to the filming sites in Cardiff and Barry Island to see the cast filming and take their own photos and selfies.
'It’s a challenge trying to keep key storylines under wraps but the cast have been reminded to be on alert.'
The BBC declined to comment when contacted by MailOnline.
It comes as Gavin And Stacey fans were sent into a frenzy when the cast were seen filming a wedding scene for the much-anticipated final episode last week.
Making a surprise return to the set was actor Steffan Rhodri - who plays Nessa's ex-fiancé Dave Coaches - casting doubt on whether Smithy and Nessa will tie the knot after five years of speculation.
He made his last appearance in Gavin and Stacey at the end of the third series when Nessa decided that she couldn't go through with their wedding.
His surprise return will no doubt spark worry from fans that Smithy declined Nessa's proposal.
Nessa, who was noticeably absent from the set was also seen without a ring on her wedding finger this week - sparking more speculation the pair don't tie the knot.
In more Gavin And Stacey news, it's been reported that the finale will see the return of Sheridan Smith as Smithy's sister Rudi, with the actress spotted filming a surprise cameo.
The star played a larger-than-life character in series one and two of the hit show and was spotted filming their scenes last week.
A source told The Sun the actress stepped right back into Rudy's shoes and it was 'like no time had passed at all'.
The source added: 'Sheridan was over the moon she could go back into the fold and her scenes are brilliant.'
'It's also been revealed that the cast is working on a behind-the-scenes programme to mark its final ever outing.
Barry Island was mobbed last week when the original cast returned to make the finale.
An emotional James told fans: 'It's all overwhelming, the fact that it's the last time we'll be here on the island. It's wonderful. It's the home of the show. It's the centre of it really, and every bit of it you're thinking: "God, I remember when we were here."'
On the first day of filming, the cast shared a picture of a clapperboard with the caption: 'The final episode of Gavin & Stacey started filming today. Truth be told, we're a little bit excited.'
It will be the first episode of the BBC sitcom since the last Christmas special in 2019. That episode, which was watched by 12 million viewers, ended on a cliffhanger with Nessa down on one knee as she proposed to Smithy.
The pair eventually marry, while their respective best friends Smithy and Nessa have a son, Neil, who is now 15 and will also return for the final ever episode.
Gavin And Stacey's final episode will air on BBC One on Christmas Day.
