Burglars raided a safe in Simon Cowell's west London home as the X Factor star and his family slept upstairs, according to reports.
Cowell and his girlfriend Lauren Silverman and their 21-month son Eric were asleep in their home in Holland Park when the burglary took place in the early hours of Friday, The Sun reported.
The burglar found a key to a safe and made off with jewellery and cash worth thousands of pounds, the report said.
The paper quoted a family friend as saying: "Obviously Simon and Lauren are really badly shaken by this. It's a horrible violation of their privacy. But more than anything they are incredibly relieved no-one was hurt.
"As upsetting as it is, Simon can cope with losing possessions, as long as his family are safe. That is his one and only concern."
A spokesperson for the Metropolitan police said that no arrests had been made but that an investigation continues.
It is the second time Cowell has had intruders in his home.
In 2012, Leanne Zaloumis, then 30, of Brownhill Road, Catford, south-east London, was given a community order after admitting smashing a window at his home with a broken brick.
In March that year, Cowell had been watching TV in his bedroom when he heard a "loud bang" coming from his bathroom and called his staff.
The woman was later found inside a walk-in wardrobe, a court heard.