The couple rung in 2014 in the Caribbean on a super-yacht off the coast of the island of Mustique.
After spending the night of the 30 December partying with A-listers including Kate Moss and Bryan Adams, they turned down Mick Jagger's party for a cosy evening in on the £25million boat on the 31 December.
The wealthy pair, who were reported to be dating six weeks after Charles' split from Nigella Lawson last July, acted "like honeymooners" at the party thrown by Kate on the 30 December.
"Charles and Trinny were acting like honeymooners – he was showing her off to everyone like a prize turkey," a source told The Sun.
"The joke they kept repeating was pointing to the yacht and laughing that they were having to 'doss down' on it."
The source added, "They joked that they were 'holed up' and were forced to 'make do' with staying aboard."
Charles and Trinny, both divorcees, sparked rumours of a romance in September 2013 after being spotted on a string of dinner dates.
Trinny, 49, split from her businessman husband Johnny Elichaoff in 2008 after ten years. She then moved on with Italian business mogul Stefano Bonfiglio, but the pair went their separate ways last Christmas.
Charles' marriage to TV chef Nigella ended in July, the month after he was photographed with his hand around his wife's neck at a top Mayfair restaurant which he has since revisited with Trinny.
In December Charles and his ex-wife Nigella were embroiled in a court case against their two former assistants who were accused of defrauding the couple.
They claimed Nigella authorised them to spend the money as she wanted to hide her drug use.
The pair were cleared of the charges after two days of jury deliberations at Isleworth Crown Court.
The verdict follows an extraordinary trial during which Nigella admitted to taking cocaine and cannabis, but denied she was a drug addict. She also told the court she had turned to cocaine after being subjected to "intimate terror" by Charles.
The sisters' barrister had told the jury that they had been "caught in the crossfire" of the fallout from Nigella and Charles' bitter divorce, and that the trial was being used by the former couple to fight each other "by proxy".