He will stay on at BT until that time and there has been no word on a successor.
In a statement, BT focused on Danon's role in turning around operations since his appointment in October 2000 and elevation to the board in November 2001.
BT chairman Sir Christopher Bland spoke of the "superb job" Danon had done and said the Frenchman had played a part in "building an exciting future for BT".
In the same statement Danon also looked back fondly at his time at the company and said he is looking forward to Capgemini, where he will be responsible for 60,000 staff worldwide.
However, the departure is bound to be viewed by some as evidence of a widely rumoured power struggle at BT over the splitting of the company and even over who will be the next chief executive, when Ben Verwaayen eventually departs.
Danon, whose remit covered expanding areas such as mobility, Wi-Fi and some of BT's growing services business, was thought by some to favour more aggressive competition by BT Retail against other internet service providers -- a move that didn't sit well with those running BT's wholesale business, which would have in all likelihood received less business from its sister unit.
There has been no word as yet on a successor to Danon. It is likely the company will look internally and externally, given its executive recruitment policy in the recent past.
In a research note, Ovum chief analyst Julian Hewett raised the idea of Royston Hoggarth, until recently chief executive at Cable & Wireless in the UK, slotting in to the BT Retail position, though the exit might also see a redefining of BT's structure, Hewett said.
Danon will have another big turnaround job on his hands at Capgemini. Despite some high-profile successes, not least the winning of the Inland Revenue outsourcing contract in the UK against incumbent EDS, the France-headquartered IT services company has had problems fully taking advantage of its acquisition of consultancy Ernst & Young.