Gavin and Stacey fan shocked after realising her wedding dress was used in Christmas finale

A Gavin and Stacey fan had the shock of their life after realising their wedding dress had been used to film the iconic finale scene.

The final episode of Gavin and Stacey aired on Christmas day, with over 12 million people tuning in to find out how Nessa and Smithy's story ended.

Some have even claimed that British TV has peaked, and that nothing will ever top the finale - written by James Corden and Ruth Jones.

A behind-the-scenes documentary is set to be released on January 1, giving fans an insight into how they made the show.

But the cast and production have already noted how difficult it was to keep certain scenes a secret.

And the return of Sonia - played by Laura Aikman - was one of the biggest secrets of all.

The actress kept everything top secret. (Instagram/lauraikman)

The actress revealed that she was required to stay in a different hotel while filming for the show, and didn't even tell her family members she starred in the iconic finale.

In an Instagram post, she also revealed that the team couldn't go wedding dress shopping to scout out her costume for the big day.

So they resorted to a bit of online shopping.

She said: "We couldn’t go wedding dress shopping (because of the secrecy), so this was from Vinted, (it probably belonged to someone who watched the show!) Shout out to our costume designer #ShanJames who had to alter it massively as 48 hrs before we shot the wedding it didn’t fit."

But the designer later confirmed that the team bought the dress on Facebook Marketplace.

Fans said the finale was the best of British TV. (BBC)

James replied, adding: "Thank you gorgeous lady! It was a pleasure to dress you an absolute pleasure even though it was a massive job altering that gorgeous dress for you.

"It was from FB Marketplace and the lady selling the dress thought I was ‘well dodgy’ when I was so secretive!!"

The lady who sold the dress, messaged James after watching the episode on Christmas Day.

She said: "I just watched Gavin & Stacey and had the oddest feeling that was my wedding dress that Sonia wore - can I ask if that’s what you used it for?"

In a shock response, the designer replied: "Hi there! I tried to find these messages yesterday so that I could let you know exactly that!

"I couldn’t say a thing as it was extremely top secret and I couldn’t take Laura to any shop incase she was spotted.

"You can now tell the world! I had to alter it quite a lot to fit her but it was worth it I think."

The seller noted that she had 'goose bumps' when she realised it was her wedding dress, and that the show was her 'absolute favourite'.