Opening Britain's postal system to private competition could leave some homes without a delivery service, MPs have warned.
Universal deliveries are 'threatened' by competition
Under new rules, private firms will compete on equal terms with the Royal Mail from January 1.
If they are particularly successful, the Royal Mail may not be able to afford a universal service, the Commons trade and industry committee said.
Postcomm, the industry regulator, said it believed the market was ready.
The Royal Mail is legally required to provide a "universal service" but MPs said they were concerned over how that would be defined.
"We emphasise that a universal service is not worthy of the name if it allows for any geographic exemptions," the report said.
It added: "The regulator must remain vigilant to ensure that greater competition in the postal services market does not come at the cost of the universal service."
Tory committee chairman Peter Luff said: "If competition is more intense than people are expecting that service could be threatened in all kinds of ways, particularly as private sector operators cream off more profitable parts of the market."
A "safety net" fund requiring private competitors to help pay for Royal Mail deliveries would "almost inevitably come too late", the MPs further warned.
Mr Luff said: "The government, regulator and the Royal Mail will have to move quickly if the universal service is in jeopardy.
"The committee is really anxious to protect a universal service similar to the one we have at present."
Market is ready
Richard Moriarty, of Postcomm, said: "We believe the market is ready - that's not just our words.
"The Royal Mail itself has said it's ready for competition, it has issued press notices to that effect.
"But also customers up and down the country, and particularly large customers, want a more competitive market and for the Royal Mail to raise its game."
He added: "Our first priority is to ensure the one-price-goes-anywhere stamp remains and that everyone up and down the country receives a daily collection and daily delivery."
Stamp prices are rising as Royal Mail battles pension deficit
Meanwhile, the committee said Royal Mail customers should not foot the £4bn pensions deficit alone.
Earlier this month, it was revealed that stamp prices would rise in April, with first class stamps going up 2p to 32p and second class stamps going up by 1p to 22p.
The possibility of them rising to 36p and 25p respectively by 2010 was also raised.
The Royal Mail had wanted to push stamps up to 39p and 27p to help pay for modernisation and to plug the hole in its pension fund.