The trip is being combined with the traditional annual visit of the Prime Minister to see the Queen at Balmoral. It is, however, unusual for the Prime Minister to appear during a by-election. Mr Blair unexpectedly campaigned on the final day of the Eddisbury contest in July only when Labour strategists mistakenly believed that the party might take the Cheshire seat from the Conservatives.
Labour is putting maximum effort into winning the seat on 23 September. Even the timing of the contest - during the SNP conference - was deliberate. The SNP hopes to benefit from the heavily criticised performance of the Labour-Liberal Democrat coalition in the Scottish Parliament and repeat Winnie Ewing's 1967 by-election victory in north Lanarkshire.
The by-election follows the elevation to the Lords of George Robertson, the Secretary of State for Defence and future Nato secretary-general.
Mr Blair's presence, backing Labour's candidate, Bill Tynan, a trade- union official, will distance the campaign from the executive of First Minister Donald Dewar.
A Labour spokesman said: "This is the first by-election in the new Scotland, which has its own Parliament. This is new political territory. What happens here will affect the way politics is handled in Scotland for the next couple of years."Reuse content