Clifford Chance advised the Ulster Unionist Party on key constitutional issues relating to the Northern Ireland peace agreement in the run-up to this month's historical settlement.
The Unionists, headed by ex-lawyer David Trimble, opted for top-notch legal advice, as they feared that changes to Northern Ireland's status would weaken the Union.
"We went to the best. We thought we needed them," said spokesman Tom Hennessy.
Clifford Chance's public international law expert Jeremy Carver advised on the legal basis of a UK-Ireland treaty, the removal of the Government of Ireland Act 1920, and the Principle of Consent which sets down that a united Ireland can only come about with the consent of the people of Northern Ireland.
Carver also met Irish government officials in Dublin to discuss the removal of the Republic's territorial claim to Northern Ireland in articles 2 and 3 of the Irish constitution.
"On the constitutional issues we won hands down," said Hennessy. "The objective was to try to negotiate from a constitutional point of view. We got everything we wanted."
The Ulster Unionist party was also advised by local barristers Peter Weir and Peter King, both party members who were elected to the Forum.
Other parties also used lawyers on aspects of the agreement. The Social Democratic and Labour Party used the Irish government's legal advisers as well as party negotiators Alex Attwood of Attwood & Co and barrister Alban Magennis, Lord Mayor of Belfast.
Napier & Co's Sir Oliver Napier advised the Alliance party on constitutional issues along with Steve McBride.
Sinn Fein was advised by Belfast firm Madden & Finnucane. Neil Faris, managing partner at Belfast firm Cleaver Fulton & Rankin, advised the Labour Group in a personal capacity, as did Brian Garrett of Elliott Duffy Garrett.
The Women's Coalition was advised on human rights by the NI Committee on Administration of Justice and inhouse solicitor Anne Marie McWilliams.