Maitland is advising TUI on UK communications, while Hering Schuppener Consulting is counselling the Hanover-based firm in Germany.
Hudson Sandler, retained as First Choice's agency since the 1990s, will be advising the other side throughout the deal.
The merger, which could save the two companies £100m a year, sees Thomson Holidays owner TUI take a 51 per cent share in the new operation, with First Choice retaining the rest.
The merged company will use the name TUI Travel.
Insiders say it is Hudson Sandler, rather than an AMO agency, that ultimately looks set to retain the corporate and financial comms brief for the newly formed company, whose CEO will be First Choice chief executive Peter Long.
Maitland partner Lydia Pretzlik currently heads the account, working alongside German-based consultant David Stürken.
The team was brought in at the start of the merger talks last month, and worked to keep the deal quiet from a usually voracious travel press.
The travel industry has had a difficult time in recent years, with consumers increasingly using the internet to plan holidays, and budget airlines cutting into profit margins. Groups have also faced an oversupply of package holidays.
The deal, creating a group with £12bn of revenues, has been flagged up as a sign of increasing consolidation within the travel sector.
British financial comms teams also advised on last month's MyTravel and Thomas Cook merger. Brunswick worked with MyTravel, while Finsbury handled the Thomas Cook account.