The Colorado-based former Liverpool co-owner had borrowed the money in 2008 for investment in the Premier League club, but has failed to pay it all back, despite it being due in August.
A 12-page legal document was filed in Manhattan's New York state court yesterday, and Mill Financial included a copy of an unlimited guarantee to pay the money back signed by Gillett.
Sued: Former Liverpool co-owner George Gillett, shown on the left alongside Tom Hicks, allegedly owes lending Mill Financial £73million
The company, based in Springfield, Virginia, reportedly refinanced a loan used by Gillett and Texas business partner Tom Hicks to buy a significant stake in Liverpool in three years ago.
According to the lawsuit his company, Gillett Football LLC, borrowed $70million (£44million) from Mill Financial in January 2008.
The loan became due on August 13 but had not been paid in full, claim the company.
In the complaint, Mill Financial state that Gillett and some of his business entities issued guarantees that 'unconditionally and absolutely guarantee[d]' the 'payment in full, promptly on demand'.
According to the lawsuit Gillett entered into a high yielding term loan that carried a 19 per cent annual interest rate.
In the month before the loan allegedly came due in August, the amount owed by Gillett Football LLC was $89.8million (£55.87million).
Mill Financial notified Gillett in August that the loan was in default and in early September Gillett Football LLC did make a $431,000 payment (£268,170), but 'this amount was not sufficient to cure the default', insist the loan company.
After added on extra fees, Mill Financial claim the amount now due is £73million. Gillett declined to comment on the lawsuit last night.
For Gillett, the Mill Financial lawsuit is the latest development in a saga that has gone on for months in courts from London to Texas.
Gillett, together with Hicks, lost their grip on Liverpool and were unable to stop the Royal Bank of Scotland, which financed their original purchase of the team, from selling the club last month to New England Sports Ventures, owned by fellow Americans John Henry and Tom Werner.
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