Wnek, who launched Ben Mark Orlando with Langdon and Orlando Hooper-Greenhill in January, said he felt "utterly betrayed" by Langdon. He added: "When I was considering doing a start-up Ben came to me unemployed. I believed in him and gave him a shot and now I don't know what to believe."
Langdon is expected to take up the position of UK group chairman, a role Wnek vacated in November to launch the new agency. Langdon, who will report to Euro RSCG's worldwide chairman and chief executive, Jim Heekin, will also be the chairman of the UK advertising agency.
Langdon admitted: "I've made a bit of a pig's ear of this one." He added: "I now know that I am better suited to the environment of multinational agencies, where I can work with big clients facing major issues ... I am not a natural entrepreneur."
Wnek and Hooper-Greenhill, meanwhile, have yet to work out their future.
BMO won a Bacardi brief last month, but it is unclear if the agency will be able to continue operating. It was also down to the final two in the Ocado pitch, but was ruled out on Monday when Wnek informed Ocado of Langdon's departure.
It has emerged that Langdon has also twice been in talks with Publicis about its top UK job, which last week went to Tim Lindsay in a deal negotiated by the headhunter Kendall Tarrant. Langdon's talks took place last November and earlier this year.
However, the emergence at Euro of his former boss Heekin, who was also removed from McCann last year, created a new opportunity for Langdon.
He was also placed by Kendall Tarrant.
Heekin said: "Ben is driven to succeed and we welcome the commitment and energy he will bring to Euro RSCG. His primary focus will be the advertising business, but he will help generate leads across the UK group."
THE STORY SO FAR
Nov 03: Langdon in talks with Publicis about senior role
Nov 03: Campaign reveals his start-up plans with Wnek
Jan 04: Ben Mark Orlando launches officially
Feb 04: Wnek and Langdon talk to Publicis about a sale
Mar 04: Tim Lindsay takes Publicis job
3 Mar 04: Wnek discovers that Langdon is talking to Euro
5 Mar 04: Langdon tells partners he wants to quit BMO
8 Mar 04: Wnek advises Ocado that Langdon is leaving
10 Mar 04: Langdon confirmed in Wnek's old job