The closure will result in substantial redundancies, though WPP has yet to confirm the exact number. The futures of United London managing partners Jim Kelly and Robert Campbell at WPP remain uncertain.
Lawrence Mellman, chief operating officer of the global United network, said: "United London has had a difficult time in the last few years. The loss of Sky was a massive body blow.
"We've taken the view that the agency is no longer viable. There will be no United branding on Grey. Clients have been informed and the same people from United will service the accounts at Grey."
Mellman refused to be drawn on the futures of Kelly and Campbell, who were parachuted into the agency in January of last year by Sir Martin Sorrell, WPP chief executive, in a last-ditch attempt to revive the agency's flagging fortunes. Each was given a 12% stake in the network.
Mellman said: "I can't categorically say whether they will be in WPP." At the time of writing, Kelly and Campbell could not be reached.
COI, Fiat UK, thelondonpaper and a number of Unilever brands are among the United London clients moving to Grey.
Mellman said the United network, which has been in much better health than its London shop, was unaffected by the decision to close its London outpost. "There will be no repercussions at the wider United network," he said.
Senior United London staff are in discussions about taking roles at Grey, including: Rhona Cairns, deputy managing director; Richard Huntington, planning director; Jason Cobbold, client services director; and Nils Leonard, head of design.
In a separate move, John Lowery, Grey London head of planning, is leaving the agency but may stay on in a consultancy role on a network level.