Simon Cowell is facing a High Court challenge over a multimillion-dollar agreement “written on a napkin”, court documents reveal.

Ian Rosenblatt, who previously served as Mr Cowell's business adviser, has initiated the proceedings.

His representative, Katherine Ratcliffe, stated in the filings that Mr Rosenblatt is seeking an agreed percentage from a confidential deal made in 2025.

Alternatively, he is claiming £100,000 in damages for an alleged breach of contract.

Mr Cowell has not yet submitted a defence to the claim.

Mr Rosenblatt, himself a lawyer, began working for Mr Cowell in 2018, Ms Ratcliffe said.

He helped with the transfer of shares in Mr Cowell’s company SyCo Entertainment, from Sir Philip Green and Sony to the former X Factor judge, leaving Mr Cowell with a 100% stake from July 2020.

Mr Rosenblatt was then appointed director of SyCo, but in late 2022 he wanted to quit.

Ms Ratcliffe said: “By September 2022 Mr Rosenblatt intended to formally resign as director of Syco, and cease providing business advisory services to Mr Cowell, because of how challenging he had found working with Mr Cowell.”

She said it was “difficult to communicate” with Mr Cowell, who was “unpredictable” and “extremely emotional”.

A month later, over dinner at Mr Cowell’s London home, the two men made an oral agreement about Mr Rosenblatt taking a percentage of a deal, Ms Ratcliffe said.

She added: “The terms of that offer were written on a napkin which Mr Cowell then signed.”

They stated that Mr Rosenblatt would receive 8% of any deal over 30 million dollars (£22 million), 10% of any deal over 50 million dollars (£37 million) and 15% of any deal over 100 million dollars (£74 million).

This was later written up and signed by Mr Cowell in February 2023 at his Cotswolds home, Ms Ratcliffe said.

Mr Rosenblatt claims however that when he sent an invoice to Mr Cowell following the deal in 2025, the fee was not paid.

Ms Ratcliffe said: “In breach of the Dinner Agreement and/or the February 2023 Letter, the fee owed to Mr Rosenblatt has not been paid within 30 days of the date of the invoice or at all.”

A spokesperson for Mr Cowell declined to comment.