The Fool's Gold actress - who is 14 weeks pregnant with her first child with the Muse rocker - reportedly held an "intimate" bash to celebrate their union with their closest friends and family at an exclusive London hotel on Monday night.
An insider told The Sun newspaper: "It was an intimate party. They kept their engagement secret as they wanted to celebrate with their nearest and dearest before it became public knowledge."
Kate's Hollywood icon mother Goldie Hawn, 65, was believed to be among the guests in attendance after having been in the UK to present an accolade at the British Comedy Awards on Saturday night.
However, the couple - who have been dating for around nine months - could be set to move back to the UK on a permanent basis after Matt, 32, revealed plans for all of Muse to move to London to make writing and rehearsing for the band's next album easier.
He told the Daily Star newspaper: "We're all thinking of moving back to London, so it'll be the first time we're all going to be in the same place since we were much younger.
"It'll make rehearsing a lot easier. Maybe in summer we'll start getting into rehearsing and writing."
Kate already has a seven-year-old son Ryder with ex-husband Chris Robinson.
- BANG! Showbiz