Manchester United confirmed today that they have secured the most lucrative shirt sponsorship deal in football, believed to be a £20m-per-season, four-year partnership with the American financial giant, Aon Corp.

United have been scouring the globe for a new sponsorship deal since their current sponsor, AIG, the American insurance company, announced it would not be renewing its £14m-a-year deal when it expires at the end of the 2009-10 season.

AIG's decision followed massive losses suffered during the current economic crisis but, despite the downturn, United are understood to have eclipsed that deal – and the lucrative sponsorships of Bayern Munich, Real Madrid and Chelsea – with an £80m agreement with Chicago-based Aon.

Old Trafford officials have held negotiations with several companies about replacing AIG in recent months, including Sahara, the Indian financial services corporation, and Saudi Telecom.

David Gill, the United chief executive, said of the Aon deal: "Today's announcement clearly strengthens our position as one of the biggest clubs in world football.

"We look forward to being closely aligned with the world leader in risk management, a firm which shares our values and is an exciting partner for Manchester United."