Celebrity chef Gordon Ramsay has told a judge that he reacted with "shock and horror" when he made the "devastating" discovery that his wife Tana's father used a ghost writer machine to "forge" his signature.
Ramsay says the machine was used without his knowledge to sign him up for the £640,000 (€800,000)-a-year rent for the York & Albany pub in London.
Ramsay is seeking a declaration that the guarantee does not bind him because his signature "was not lawfully authorised" when the 25-year lease was signed in 2007.
He says for a number of years he worked with his father-in-law, Christopher Hutcheson, who acted as business manager for his group of companies.
Ramsay told the court in London that his "deep trust in Hutcheson was entirely misplaced" and his father-in-law defrauded him and the group "of hundreds of thousands of pounds".
In October 2010, Mr Hutcheson was "summarily dismissed from the group".
As Ramsay was "piecing his business back together", his solicitors discovered his signature had appeared on a personal guarantee in the lease for the York & Albany.
Ramsay told the court that discovering the existence of the lease guarantee was "a shock - a devastation".
"The company is still in the hook for the lease, and what was devastating for my wife and I was that we were guaranteeing it until 2033," he said.
The court was told a key issue in the case was whether Mr Hutcheson was authorised to use a "ghost writer" machine he had obtained and which was used to sign Ramsay's name on the guarantee.
Film director Gary Love, who owns the York & Albany, has described Ramsay's allegation as an attempt to wriggle out of his rental commitments.
Ramsay is submitting to the court that he had agreed to the use of the ghost writer, which electronically replicates a signature using a pen.
But he had done so "for purposes of merchandising material only".
Ramsay says Mr Hutcheson used it to sign the York & Albany guarantee or loan, even though the chef had not authorised anyone to sign it on his behalf.
Ramsay said that between six and nine months after Mr Hutcheson was "removed from the business" in 2010, "it was discovered that the lease was signed by the ghost machine".
Under cross-examination, Ramsay said his father-in-law "never had permission" to sign any deal "via a ghost machine".