Homeowners who have not missed a single mortgage repayment could still be threatened with repossession.

The warning came after Royal Bank of Scotland (RBS), which is majority-owned by the Government, wrote to customers Peter and Marian Addyman insisting a £226,000 mortgage was paid back within 30 days. The letter threatened repossession if the cash was not found, even though the couple have not missed a single mortgage repayment.

NatWest, which is owned by RBS, has refused to explained why the mortgage was being withdrawn.

The Council of Mortgage Lenders (CML), the trade body which represents UK mortgage lenders, said yesterday that a clause allowing lenders to demand that a mortgage is repaid at short notice exists in the small print of almost every mortgage issued in the UK.

However, it said that the clause was only meant to cover exceptional circumstances.

Consumer groups said they were shocked by the behaviour of NatWest.

Peter Vicary-Smith, chief executive of Which?, the consumer association, said: “How can a family that has seemingly done nothing wrong face losing their home without any explanation from the bank?

"The customer has a right to know what the problem is before they’re threatened with losing their home. All that has been said is that there are exception circumstances surrounding the case, but it has not explained what these are. Other homeowners are bound to worry that they may also fall into the same category.”

Gordon Brown has told lenders that repossession must be a “last resort” and has put in place new rules ensuring that courts insist that every possible alternative has been explored before granting a repossession order.

At the beginning of this month, RBS announced that it would wait six months before threatening a customer who was in arrears with repossession.

The CML expects a huge surge in the number of people who loose their homes. It predicts that 75,000 homes will be repossessed next year and half a million homeowners will be at least three months in arrears on their mortgage repayments.

The Financial Ombudsman Service is examining the case of the Addymans, who live in St Leonards-On-Sea, East Sussex with two sons aged 12 and 15.