Senior cabinet figures said that the decision to keep Mr Robertson in the Cabinet until after he takes up his new post as Secretary-General of Nato was the clearest signal that the Prime Minister is planning sweeping changes which he ducked in July.
"He's planning a big reshuffle. He can't do that before the party conference [in October]," an insider said. Two Blairite ministers, Tessa Jowell, the Public Health minister, described as "ultra loyal" to Mr Blair, and Geoff Hoon, a Foreign minister, were last night being tipped for promotion to the Cabinet. There was also the inevitable speculation of a return for Peter Mandelson and a move for the Northern Ireland Secretary, Mo Mowlam.
Mr Blair was criticised for making no cabinet changes before going on his summer holiday, but delaying the reshuffle was seen as a signal that he does not intend a simple replacement of Mr Robertson with John Reid, the Scottish Secretary, who wants the job.
The Prime Minister's decision to give Mr Robertson a peerage on his promotion to Secretary-General of Nato has enabled Labour to force a snap by-election in his Scottish seat on 23 September, leading to allegations that he was abusing the peerage system. The by-election will coincide with the first day of the SNP's annual conference, and limits the party's scope for mounting an effective campaign for the constituency.
Labour has also called a by-election on the same day in Wigan for the vacancy caused by the recent death of Roger Stott, the well-liked Labour backbencher and former Northern Ireland spokesman.Michael Ancram, the chairman of the Conservative Party, said: "This is a clear abuse of the peerage and exposes Tony Blair as a cynical opportunist." The Labour Party denied SNP claims that it was "running scared" of the by-election, but admitted that the move would give Labour the edge in the battle for the seat, where Labour has a majority of 15,878.
Awarding a peerage to Mr Robertson allowed him to avoid having to wait for the Commons to return in October from its long summer break before stepping down as an MP. It will also keep him in the Cabinet until the Prime Minister decides on another reshuffle. Labour has adopted a local candidate to fight the seat, and cited a precedent in the Tories' elevation to a peerage of David Waddington to become Leader of the Lords in 1991.
John Reid, the recently appointed Secretary of State for Scotland, who is the leading candidate for Mr Robertson's defence portfolio, insisted that the SNP was "divided and disrupted".
Mr Robertson, who takes the title of Lord Robertson of Port Ellen, a village on the Hebridean island of Islay where he was born, will not be taking the Labour whip during his time as Secretary-General of Nato.Reuse content