HUNDREDS of post offices face 'stealth' closures as a result of the controversial privatisation of the Royal Mail, claims a report by a left-wing movement.

The Fabian Society said there was an increased likelihood the Royal Mail would want to renegotiate its contracts with the Post Office following the sell-off.

The business accounts for a third of the Post Office's annual revenue, but if mail revenues decline, that could be cut by more than a fifth - £278 million - said the socialist think-tank.

Reductions of this magnitude would have a "potentially disastrous" effect on Post Office services, especially as subpost­masters are already seeing their income fall, said the report.

Citizens Advice Scotland said it would be "very concerned" about any closure programme.

Spokeswoman Sarah Beattie-Smith said: "Post offices play a vital role in communities, and CAB advisers see that first-hand in many of cases they deal with.

"Many people use the post office as a one-stop shop to collect pensions and benefits, pay utility bills, buy stamps etc.

"We have done surveys that have shown people feel safer collecting money and paying bills in one venue, than carrying their money from place to place to pay accounts. This is particularly the case with elderly and/or disabled people, and people in rural areas."

Natan Doron, senior researcher at the Fabian Society, said: "The Government has repeatedly insisted the future of the Post Office network is separate from the debate about the privatisation of Royal Mail, but this argument does not stack up.

"The Post Office relies heavily on the contract it has with Royal Mail. Any renegotiation puts the network of post offices that people rely on in serious jeopardy.

"The loss of majority control of Royal Mail to the public means any attempts to change the terms of agreement with the Post Office will be impossible to stop."

She said it was important the UK Government retained its stake in Royal Mail to give it the option to buy back enough shares to regain control in the future. Ms Doron added: "Failure to do so could result in hundreds of local post office closures by stealth."

George Thomson, general secretary of the National Fed­eration Of SubPostmasters, said: "How many warnings does this Government need before it sits up and takes action to protect the thousands of independently-run post offices that underpin so many communities?

"The Fabian Society's report spells out in detail what has to happen for the network to survive, but sadly concludes a positive outcome is unlikely. We support this belief."

The report was published as hundreds of managers at Crown Post Offices staged a 24-hour strike in a dispute over pay.

Unite, which represents 900 managers, said the action hit services at the offices, the larger branches usually sited on high streets.

The union said its members had not received a pay rise since June 2011 and accused the Post Office of "dragging its feet".

Unite official Brian Scott said: "While we understand the industrial action will cause inconvenience to the public, the ball is in the management's court - as it has been for the last 16 months - to reach a fair and equitable settlement."