The former glamour model - who recently discovered her husband was having a ''full blown sexual affair'' with her married best friend - didn't pick up on the spark between the pair, according to a house guest.
Jemma Henley - who spent four months at their home - told The Sun newspaper: ''To get attention she would laugh at Kieran's jokes and stroke his arm - you could cut the sexual tension with a knife.
''There were looks that went unseen by Katie - but I didn't miss them. How she didn't see this coming is beyond me. Jane was always giggly around Kieran and she didn't try to hide it.
''But if there is one person Katie trusted it's Jane - she could do no wrong in her eyes. Maybe she left them alone one too many times. Jane was once me she liked his muscles.''
Jemma explained how Katie and Jane were ''like sisters'', but thinks the star's bridesmaid was influenced by ''jealousy''.
She said: ''There were like sisters. If there is one person Katie would die for it's Jane. I really feel for Katie in all this.
''These are the two people closest to her. This is the ultimate betrayal ... Jane has lived in Katie's shadow for 12 years - that's a long time to play second fiddle.
''She was jealous and that has led to her bedding Kieran.''
The '12 Years A Slave' director will receive the accolade at the London Film Festival in October.
Critics from all over the world were asked to name the best movie of the past 16 years.