Fears have been expressed over the six-day-a-week postal guarantee despite the Conservative Government insisting the pledge is not under threat from plans to fully privatise the Royal Mail.
Unions, opposition politicians and former cabinet ministers warned that the Chancellor George Osborne's proposals could put the service, crucial to many rural communities, in jeopardy.
The Government said there can be be no change to the Universal Service Obligation (USO) because it was now enshrined in law.
However, Alistair Carmichael, the MP for Orkney and Shetland and former Scottish Secretary, said: "The Government owning 30 per cent of Royal Mail was an important part of the package put in place by the Coalition Government to protect the USO.
"If that element is to be removed then the whole package will require to be looked at again."
He added that "the most important protection for the USO is the power of the regulator to charge a levy on private operators who do not deliver to every door in the country, as Royal Mail does.
"I shall be urging ministers to ensure that, before the sale goes ahead, there is a comprehensive review of the powers of the regulator to protect the universal service."
Union leaders also expressed anger that no shares would be reserved for employees - who received 10 per cent last time around.
The sale comes just two years after a botched part-privatisation of the Royal Mail.
A probe by MPs later concluded that taxpayers had lost out on as much as £1billion from the deal.
The Chancellor said the government wanted to sell the remaining 30 per cent stake in the Royal Mail, in a bid to raise around £1.5 billion.
The USO ensures the delivery of letters and parcels to any address in the country every day of the week except Sunday for the same price.
Under pressure over the impact of part-privatisation, ministers wrote the USO into law.
But critics warned at the time that future governments could amend that obligation.
The SNP's Business, Innovation and Skills spokesman Michelle Thomson accused the Conservatives of not being straight about their plans in the run up to last month's election.
She said: "This is a sleekit manoeuvre by George Osborne - which he and David Cameron kept very quiet about during the General Election campaign.
"The UK Government's decision to privatise the remainder of Royal Mail poses real danger to the postal service."
She warned: "A privatised postal service in a competitive market will undoubtedly put pressure on government to reduce the terms of the USO, as its competitors cherry pick the profitable urban routes leading to a reduced service in rural and less affluent areas.
"It threatens to have particularly harmful consequences for Scotland, where so many of our rural communities and so much of our rural economy rely on the Royal Mail service."
Labour's shadow Scottish secretary Ian Murray said the move exposed the Conservatives "true colours".
He added: "Labour will oppose any attempt to sell off the remaining public share in Royal Mail.
"It is an essential asset that should have remained in public hands. For the sake of families and small businesses across Scotland the government should retain its stake."
Mr Osborne told MPs: "It is not our money or the Government's money, it is the money that people earn and pay in taxes".
The Communication Workers Union (CWU) general secretary Dave Ward said the announcement "shows the Tories true colours."
He added: "Selling off the final 30 per cent of Royal Mail threatens the very existence of the one-price-goes-anywhere, six-day delivery service that Royal Mail provides to 29 million UK addresses."
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