The Scottish National Party is to bring forward a member's bill on proportional representation for local government.

The surprise move comes after the first minister in waiting, Jack McConnell, promised the Liberal Democrats he would deliver a timetable on electoral reform by Christmas.

The way is clear for Mr McConnell to become first minister, after he secured the support of Lib Dems MSPs with a pledge on voting reform for local councils.

Scottish National Party Leader John Swinney, independent MSP Dennis Canavan and Tory leader David McLetchie will challenge Mr McConnell for the post of first minister.

Tricia Marwick: Expected to unveil SNP plans

Scotland's Labour leader promised a timetable for electoral reform by Christmas, but the SNP is set to increase pressure on the coalition.

The nationalists will outline how they intend to take forward the campaign to see legislation on proportional representation elections for councils.

SNP local government spokesman Tricia Marwick is expected to unveil plans for a member's bill to introduce PR.

The party said the time for talking has passed and it is now time for action.

This new development could cause a dilemma for Lib Dem backbenchers, who are concerned that progress has been slow.

Before Tuesday's meeting Mr Swinney attempted to increase the pressure on Lib Dem leader Jim Wallace by saying that he needed to "show some backbone and insist on a firm commitment to PR for local government".

Mr Wallace said that although the Lib Dems recognised that those within the Labour Party had different opinions on reform, progress had to be made.