With the help of The Kills frontman, the 37-year-old beauty called for first aid volunteers who gave her an oxygen mask.
"Halfway through, she was feeling faint. The place was jumping and she had to get Jamie to help her to the first aid room."
After St John Ambulance volunteers assisted her backstage, Jamie took Kate back home before setting off without her to carry on his night out.
The source added: "Afterwards he took her home in a taxi and then went out to carry on the party at a London Fashion Week bash."
Earlier this month, Kate - who is due to marry Jamie on July 2 - fainted during a drunken day out with friends.
The blonde beauty visited a London sex shop and after buying seven sex toys and sniffing some poppers she passed out in the x-rated store.