The decision to afford Mr Robertson a life peerage before his move to the Nato secretary general's job in October means a by-election in his Hamilton South constituency on 23 September.

SNP South of Scotland MSP Mike Russell: "It's a political fix"

The move has infuriated opposition parties who have accused Labour of flouting parliamentary convention by calling a by-election in the Westminster recess.

And it has prompted allegations from the Scottish National Party that Labour is seeking to wrong foot them by announcing the poll will take place on the first day of their party conference in Inverness.

The row adds spice to what was always going to be a bitter fight for the most prized of Scottish seats.

The SNP aims to turn back the clock

A victory in Hamilton South would be seen as a precious scalp for the nationalists.

In 1967, Winnie Ewing swept to an historic victory in what was then the Hamilton constituency, rejuvenating the nationalist movement.

Mrs Ewing held the seat for two-and-half years before Labour regained it.

If it is to repeat Mrs Ewing's victory, the SNP will need to overhaul Mr Robertson's sizeable 16,000 majority in a seat he has held since 1978.

The SNP have yet to choose a candidate to oppose Labour selection Bill Tynan but Mrs Ewing's daughter Annabelle is believed to be favourite.

SNP South of Scotland region MSP, Mike Russell, said Labour had broken convention by announcing a by-election during a party conference and offering a peerage to someone before they had completed their international duty.

But he stressed that the nationalists are ready for the fight.

'Political trickery'

He said: "This is a fix, it's a piece of political trickery, it's dishonest, it's the usual New Labour approach to life.

"We will fight the by-election vigorously and enthusiastically because we always do but it is not the way to handle politics and this is New Labour at its absolute worst."

Allegations of political manipulation were echoed by Conservative constitutional spokesman Michael Ancram.

He said: "This really is unseemly haste and Labour know this is the only way they can do this during an recess unless the incumbent has died.

"The reason why Labour are going to the constituency so fast is they know that every day that passes people are beginning to see that this government is not delivering."

Dr John Reid: "Fitting reward"

But Scottish Secretary Dr John Reid dismissed the opposition assertions and said Mr Robertson's ennoblement should be seen as reward for his contribution to Britain and not a political trick.

He said: "George Robertson has been recognised internationally by all the major European countries and the Americans by the post he has been given.

"And it is perfectly fitting and proper that someone be promoted to the House of Lords on merit."