Labour has launched a campaign against the Government's controversial plan to privatise the Royal Mail, branding the move "dangerous".
The party leadership is throwing its weight behind growing opposition to the sell-off.
Royal Mail managers this week voted strongly against privatisation, while the Communication Workers Union is threatening to ballot its members for a strike unless there is a deal on pay, pensions, jobs and other issues linked to the sell-off.
Labour MPs will stress that the public and businesses rely on a daily delivery service and will warn that places where it is expensive or complicated to deliver the post every day - like rural areas or blocks of flats - could be "vulnerable" if privatisation goes ahead.
The party is also warning that many customers might have to travel miles to pick up large parcels as a privatised Royal Mail could choose to sell off its assets such as local delivery offices.
A Labour source said: "We all rely on the Royal Mail - from receiving that parcel we've ordered online to knowing that birthday card we've sent is going to arrive in time - but some of us rely on it in different ways.
"In rural areas the local post office acts as a focal point for communities and there are many pensioners across the country who are reassured by seeing their regular postie every morning. There are also the small businesses that use the Royal Mail for ensuring customers get their goods and services on time, efficiently, reliably and at a reasonable price.
"Which is why the Tory-led Government's plans for privatising Royal Mail are so dangerous.
"Like so many of this Government's plans, these haven't been thought through properly. They want to sell off Royal Mail on the cheap in order to plug a financial hole in the British economy caused by George Osborne's failed policies as Chancellor."
Labour believes privatisation could have "enormous consequences" for those who rely on the postal service and accuses the Government of being unwilling or unable to say what they will be.
Privatising Royal Mail could mean that profit comes before the needs of communities, Labour warned.
Local post offices rely on the Royal Mail for much of their business and many of them could be under threat if privatisation goes ahead as they look to slash costs, while organisations up and down the country have warned of higher prices and a lesser service, Labour warned.
The source added: "David Cameron doesn't need to do this. The Royal Mail made a £400 million profit last year but there's a real risk that this money will go to private shareholders instead of being invested back into the service."
A Department for Business spokesman said: "This cynical scaremongering will not succeed in stopping our plans to ensure Royal Mail can thrive and deliver a better service to consumers and business. Regardless of ownership, Royal Mail will still be the UK's designated universal service provider and will continue to provide deliveries to all UK addresses - rural and urban - six days a week, only Parliament can change this.
"Royal Mail is one of Britain's biggest companies but it also needs future access to private capital to be able to continue its modernisation programme and to seize opportunities for growth such as the boom in on-line shopping.
"That is why we are pursuing a sale of Royal Mail shares this financial year. Parliament decided two years ago that this was the right approach; it would be hard to say we have rushed the process.
"The Post Office is not for sale and there will be no programme of closures under this Government. The Post Office is independent of Royal Mail but they are natural partners and that is why a 10-year long term commercial agreement between them for the continued supply of services was signed in 2012."
A Royal Mail spokesman said: "Royal Mail is honoured to provide the universal service to more than 29 million addresses across the UK. The universal service is enshrined in law through the Postal Services Act 2011.
"The six-days-a-week, one-price-goes-anywhere, affordable service can only be changed by both Houses of Parliament. This includes the provision of services to rural areas. This is the case whether Royal Mail is in the public or private sector. Ofcom, the independent regulator, has a primary duty to protect the universal service. It has ruled out any changes to the scope of the universal service."
A Conservative party spokesman said: " Conservatives are taking action so that Britain's six days-a-week, one-price-goes-anywhere service can survive.
"By contrast, Labour had 13 years to put Royal Mail on a sound financial footing, but failed to do so.
"Now, the same old Labour Party oppose giving Royal Mail access to the private capital it needs, meaning more government debt.
"Clearly, their policy has been bought by trade union bosses. If Ed Miliband is too weak to help Royal Mail get the investment it needs, he's too weak to run the country."
Len McCluskey, general secretary of Unite, said: "The Royal Mail sell-off is nothing short of daylight robbery. The Government is doing what even Margaret Thatcher didn't dare do in selling off the Queen's head to pay for George Osborne's economic failure.
"It is a deeply unpopular move and voters will not be fooled. They know that privatisation is bad for business and is likely to lead to rising prices and an end to the universal postal service which so many communities rely on.
"The move also spells a future of uncertainty for hardworking staff who have made the Royal Mail the success it is today.
"The Government needs to ditch the unpopular sell-off of a national institution whose roots date back to Oliver Cromwell."