Peter Kenyon has left his job as Manchester United chief executive to take the same position at their English premier league rivals Chelsea, Kenyon's solicitors said in a statement on Monday.
The appointment of Kenyon, who is being replaced at United by David Gill, represents a boardroom coup for Chelsea following their unprecedented 110 million-pound ($174.4 million) player buying spree since Russian billionaire Roman Abramovich took over the club in July.
"Peter Kenyon has recently been offered the job of chief executive officer of the Chelsea group of companies, a position that he has accepted," said the statement issued on Kenyon's behalf by his solicitors Mishcon de Reya.
"Peter Kenyon has been employed by Manchester United for over six years. He has enjoyed his time at the club enormously.
"However, the new opportunity that has arisen at Chelsea represents a challenge that Mr Kenyon feels is right to take at this stage of his career," the statement added.
A statement from Manchester United said Gill will take over as chief executive at Old Trafford, adding that Kenyon would go on garden leave in accordance with the terms of his contract.
"We would like to thank Peter for his significant contribution to the success of Manchester United on and off the pitch in the last six years," Manchester United PLC chairman Roy Gardner said.
"David (Gill), as Managing Director, has also played a major role in that success, and we are delighted that his appointment will bring both experience and continuity to this position."
The 46-year-old Gill joined United in 1997 as finance director and was appointed deputy chief executive in August 2000.
Kenyon took over as United chief executive from Martin Edwards in August 2000 after joining the club in 1997 from sportswear manufacturer Umbro International.
He had a strong relationship with United manager Alex Ferguson, signing several world-class players including Argentina midfielder Juan Sebastian Veron, Dutch striker Ruud van Nistelrooy and England defender Rio Ferdinand for a combined fee of about 80 million pounds over the past two years.