The prime minister met Mr Dewar for 45 minutes before going on to talks with business leaders in which he told them Scotland was the place to be in terms of inward investment.

Scottish Secretary John Reid also attended the meeting.

BBC Scotland's David Porter reports on Mr Blair's visit

In the past prime ministers have avoided getting directly involved with by-election campaigns, but Mr Blair's aides say it shows he is not afraid to face questioning from the public or political rivals.

But rival parties are lining up with a series of challenges to Labour's policies - both north and south of the Border.

Fuel costs

Mr Blair is due to spend the weekend at Balmoral, which has prompted the Scottish National Party to target him on the cost of fuel.

The SNP claims his own journey from Hamilton to Balmoral would be prohibitively expensive for many Scots.

The Tories are focussing on Labour's record in the Scottish Parliament, saying the new executive has proved a flop.

And the Liberal Democrats are challenging Labour to expand public spending - while curbing the tax burden on the poorest in society.

Tories' choice

The Conservatives have chosen Charles Ferguson, 35, a solicitor advocate, as their candidate.

Mr Ferguson, who is married with six children, stood for the Scottish Parliament in the Strathkelvin and Bearsden constituency.

He is chairman of the Scottish Conservative Lawyer Criminal Committee.