The star - who co-wrote the sitcom - starred as Stacey's best friend Nessa in the hit BBC series.
When asked whether the show will come back, Ruth said: "It's not coming back."
She added: "I'm really hugely complimented that people love it so much. But it is quite an extraordinary thing...it's almost like you mention the words Gavin and Stacey and people assume it's back."
However, she reiterated that 'it's not.'
On the bright side, Ruth revealed she was 'sure' she and co-writer James Corden would work together again.
"I'm sure, at some point, James and I will write something together," she said to The Independent.
"But it won't be Gavin & Stacey. Certainly, when he's back in the UK, I would have thought we would try and get together and write again because it was such fun to do that."
The show is such a huge hit that it's been the subject of rumours for many years since the original show last aired in 2010.
A special aired on BBC One on Christmas Day in 2019, which delighted fans and added more fuel to the fire.
Earlier this summer, James - who played Essex lad Smithy - was asked about the possibility of the show returning.
Speaking on BBC Radio 2's Breakfast Show, temporary host Vernon Kay asked: "Gavin & Stacey – is it the end or are you sitting down writing or is the story going to continue? What’s going on in your head?"
James responded: “I wish I could say it’s up to me. The best thing about Gavin & Stacey is that it’s Ruth and I and, if I’m being completely honest, her judgement is exceptional.”
Vernon added: “Right, so she’ll know when will she?”
James explained: “Yeah, or if. And so, look I am going to have a bit more free time as of sort of next year and that will open some time for us to just be together I hope, but honestly, and I’m so touched every time I get here when people ask about it.
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"It’s amazing, it's incredible how people care about those characters in the show. It’s so great and I realise the ending of that special does very definitely tease something else. But, Ruth is, it’s always been the two of us, it’s always been.
"And we can’t write separately. We’ve never been able to write over Zoom or anything at all. We’ve just got to be in a room together, talking with a pack of Post-It Notes and a pen and at some point we get a laptop out, but that’s even way down the line. And she just always knows."
Other stars of the show are just as devoted to Gavin & Stacey.
Larry Lamb, who plays Mick Shipman, was previously asked if he was down for another special.
"Yeah, I think everyone that's involved in it. It's a part of your life, and you're a part of it. Yeah, definitely," he said.
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