Clifford Chance won a historic mandate to help the Balkan state of Montenegro secure its independence from Serbia.
The firm's head of public policy Michael Smyth and international law head Jeremy Carver were instructed alongside a local firm to provide advice on the recent referendum.
The vote, held on 21 May, saw a massive 86.3 per cent turnout as Montenegrins made their voices heard about the future of their country. Many expatriates flew home to register their votes.
A majority of 55.5 per cent of the votes were in favour of independence, meeting the threshold requirement set by the EU after consultation with the government and opposition.
Clifford Chance provided advice on both the criteria for the referendum and the legal consequences if a vote in favour of independence was more than 50 per cent, but fell below the 55 per cent threshold.
Smyth said: "This is blue-sky thinking. Weighted majorities are not unique - they've been a feature of other referendums, including the referendum that took place on Scottish devolution in the late 1970s."
Montenegro was previously part of Yugoslavia and became tied to Serbia in 2003 after the republic's break-up.
The state formally celebrated its independence on 3 June.