Willie Rennie met Speaker Michael Martin on Monday

Mr Rennie caused political shockwaves when he overturned a huge Labour majority to win last week's Dunfermline and West Fife by-election.

Mr Rennie took the oath of allegiance and signed the roll of the House on Monday before shaking hands with Speaker Michael Martin.

His arrival cuts the UK Government's notional majority to 64.

Crucial votes

Mr Rennie's seat, which neighbours Chancellor Gordon Brown's constituency, was made vacant following the death of Labour's Rachel Squire in January.

Mr Rennie saw off the challenge of Labour's Catherine Stihler who came second, with the SNP's Douglas Chapman third and Tory candidate Carrie Ruxton fourth.

A small crowd of jubilant Lib Dem supporters had welcomed him as he arrived at the Palace of Westminster.

He arrived on a day crucial for the government amid a possible rebellion over its identity card scheme.

The Lib Dems, Tories and SNP were expected to oppose the scheme.

Later this week, MPs will vote on the government's new anti-terror legislation, on which the prime minister has already suffered two damaging defeats in the Commons and could face a third.