His retirement leaves only eight of the victorious 22-man squad from 2003 still playing professional rugby.
He also reached the heights in the domestic game, starting at fly-half in last season's Premiership final as his London Irish side lost 10-9 to Leicester.
Catt also won four league titles with Bath and helped guide the club to its only Heineken Cup triumph with a 19-18 win over French side Brive in 1998.
He has made 95 appearances for London Irish, the last of which came in the 38-0 win over Sale on 28 March.
Catt, who won the club's player of the season award in 2006, had intended to retire at the end of last season.
However injuries to fly-halves Chris Malone and Jamie Lennard at the start of this campaign persuaded him to stay involved on the pitch.
The signing of Crusaders' Daniel Bowden, who can play fly-half and inside centre, should compensate for Catt's absence from the Exiles' backline next term.
A knee injury to Seilala Mapusua means he is likely to be given a chance to bid a final farewell to the Madejski Stadium against the Saints.
"London Irish has been privileged to have Mike Catt wear its colours for the past six seasons," said head coach Toby Booth.
"He is an inspirational sportsman and we look forward to working with him as he continues the transition into what we are sure will be a very successful coaching career with us."
Catt has recently distanced himself from suggestions he could take over the director of rugby job at relegated Worcester recently vacated by Mike Ruddock.
Rugby Football Union director of elite rugby Rob Andrew, who played alongside Catt for England in the mid-1990s, led the tributes to the veteran.
"Mike won his first cap against Wales in 1994, and by the following year he was an established and invaluable member of the squad, helping England to a third Grand Slam win in five years in 1995," said Andrew.
"When I retired from playing for England in 1997 I'm not sure what answer I would have given if I'd been asked would Mike still be playing for England in 2007!
"As we all know, he did just that, picking up a runners-up medal to add to his winners' medal from 2003, and leading his country in the same year.
"Mike is undoubtedly a legend of the club and international game. and his vision and coaching expertise is there for all to see now that he's at London Irish working with Toby Booth."
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