The 36-year-old is thought to have watched in secret for 10 minutes before confronting her 27-year-old husband and then-best friend Jane, 50.
While Katie previously stated she only witnessed the unfaithful pair kissing while they were on holiday, an insider claimed she made the new revelation during her speech at her recent wedding vow renewal ceremony.
A source told the Daily Star: “She had her suspicions on holiday, so when she noticed they were both missing she went searching. She eventually wandered down to the beach and got the shock of her life.
“They were having full-blown sex on the sand in front of her. Katie was pregnant with Kieran’s baby yet there was her husband and best friend having sex in front of her.”
Mum-of-five Katie has spoken candidly of Kieran and Jane’s betrayal in the past, but this is the first time it’s been claimed she caught them in the act.
Speaking to OK! magazine recently, Katie said guests were shocked by her speech for Kieran during their second wedding.
She said: “It was quite dramatic! My speech started by saying, ‘Some of you are probably wondering why we’re sitting here about to renew our vows after only being married two years.
“Then I said, ‘This is why,’ and I basically summarised everything that’s happened, him sleeping with my best mates when I was pregnant… I didn’t hold back!”
Following her sensational revelation about Kieran and Jane last May, Katie exposed another one of his affairs with former friend Chrissy Thomas later that year.