Wealthy borrowers are being given as little as one month's notice to pay £1m off their mortgages, as banks take ever more dramatic steps to cope with the housing downturn.

Charles McDowell, a prime property consultant in London, told the Financial Times that lenders had asked a number of his clients with large buy-to-let property portfolios to come up with more cash after falling prices slashed the level of their equity. NatWest had asked the owner of a £5m property portfolio in London to hand over £1m in a month to compensate for a 20 per cent drop in value, even though there was no evidence the client was likely to default on the loan.