SJ Berwin partner and former Liberal Democrat candidate Philip Goldenberg was called in by the party to help devise the legal framework for last week's move to provide the Lib Dems with their first real voice in Government.
Before the election Goldenberg, who stood and lost against Conservative MP Humfrey Malins in Woking, Surrey, advised his party not to take a seat in cabinet if one was offered by Tony Blair, but to push for seats on a cabinet committee instead.
"The established constitutional position is that if you are part of the cabinet you have collective responsibility for all its decisions," he said.
"That means that the minute you join the cabinet it is difficult to distinguish yourself from the governing party and to criticise it."
He said that by joining a cabinet committee, the Lib Dems would not be bound by the doctrine of collective responsibility and would be free to criticise the Government on policy issues outside electoral reform.
However, members of cabinet committees, coming under the aegis of the Privy Council would still be bound by the Official Secrets Act.
Chaired by Tony Blair the voting reform cabinet committee that was announced last week is expected to include five prominent Lib Dem MPs, including Paddy Ashdown and the president of the party, Robert Maclennan.
Before the election Maclennan and then trade and industry spokesman Robin Cook agreed to set up an electoral reform commission to hold a referendum on proportional representation. The new committee's first task will be to appoint the commission and draw up a timetable for the referendum.
Goldenberg is a long-standing voluntary legal adviser to the Lib Dems. In the late 1980s he advised the Liberal Party on merging with the SDP.