The revelation that an £8 million deal has been sealed with TNT Post to deliver mail for 89 public sector bodies across Scotland, including hospitals, councils and colleges, comes as last-ditch talks between Royal Mail management and trade unions to avert a nationwide postal strike tomorrow showed no signs of success.

Commenting on the decision to hire TNT, Alistair Carmichael, the Liberal Democrats’ Scottish spokesman, said: “Without business of this sort, the Royal Mail will become unsustainable.”

The Orkney and Shetland MP described the contract as a “false economy” because, ultimately, ordinary people would have to pay more for their daily delivery service through higher charges levied by either a struggling Royal Mail or a private-sector provider.

“This will be bad news for communities that rely on a daily service to their doors six days a week. It is another nail in the coffin of the Royal Mail,” he added.

However the Scottish Government stressed the agency, Procurement Scotland, which agreed the deal, was set up last year “to secure the best possible value for public funds”.

Its spokesman said the agency had awarded two contracts for mail services covering most of Scotland’s public bodies. A £17m first-class mail service contract has been awarded to Royal Mail but the £8m second-class mail contract has gone to TNT Post.

The spokesman added: “This will save the public purse £2.95m annually.”

Prior to the establishment of Procurement Scotland, public sector bodies negotiated their own postal services agreements.

Earlier at Westminster, Lord Mandelson urged the unions to draw back from the brink of their “totally self-defeating” two-day strike action. The Business Secretary warned that if Royal Mail failed to modernise it would face “terminal decline”. Several retailers, including Amazon, have already said they will use other carriers in the event of a strike.

Last night, a spokesman for the Royal Mail in Scotland made clear it had put in as competitive a bid as possible and was “trying to retain as much business as we can in a competitive market where competition is fierce”. TNT Post declined to comment.