The account win for the Government's marketing arm is one of the biggest in Carat's history and places the Aegis-owned agency at the top of the billings list for television, pushing Starcom down into second place.
This fight for the third largest TV account was one last hurrah for Carat's former head of media Mark Jarvis, who played a key role on the pitch before his departure to Carat client Gizmondo.
Peter Buchanan, COI deputy chief executive, said: "The pitch process was prolonged because it was such a close competition. On this occasion, Carat had the competitive edge and now that we have a result, we look forward to working with them."
Nigel Sharrocks, chief executive of Aegis in the UK and Ireland, said: "I'm glad the quality of our work has won through."
Last year COI spent £77.6m in TV billings, more than its £69m average for the past three years.
The new contract is expected to take effect from June, following a short handover period.
And for the first time the Department for Transport has announced that it will now work with the COI's media team. Media Week understands that the inclusion of the DfT, one of the decision makers in the pitch process, could have swung the decision Carat's way.
Starcom's television billings will drop from £460m to £382m with the loss of the COI account, while Carat's TV business jumps from £342m to £420m.
Starcom chairman Jim Marshall admitted that the loss of the COI TV account was a blow to the agency, but denied that its place in the TV billings list was a major priority.
"At the end of the day they've gone with Carat and on that basis they appear to have made a better offer," said Marshall. "It's a big billing figure. But we're talking a TV assignment, so in revenue terms there is proportionally a less marked impact.
"It's quite a blow for the team".