Homeowners in the north of England, Wales and Scotland are the most likely to suffer if there is any Brexit-triggered collapse in the housing market, says a credit rating agency. House prices have risen most quickly in London and the south-east of England, leaving borrowers in these areas least exposed to the risk of negative equity, said Moody's Investor Services.

If house prices were to decline by an average of 20% amid the economic and political uncertainty surrounding Brexit from the EU, Moody's calculates that around 22% of northern borrowers would fall into negative equity compared to just 1% in the south. A 10% drop would leave 9% of northern homeowners in negative equity compared to just 0.03% in the south.

"Post-crisis price gains have not been even across the country, placing borrowers in northern England, Wales and Scotland at greater risk of negative equity if house prices were to decline," says Steven Becker, an analyst at Moody's.

A Treasury analysis of Brexit released before the 23 June referendum, in which Britain voted to leave the EU, said a severe economic shock could send house prices crashing by as much as 18%. Housebuilder share prices have fallen sharply since the vote over concerns that weakening consumer confidence and a mini-credit crunch will nobble housing demand.

Since the crash, house prices have recovered to their pre-financial crisis levels. In London, where there is a protracted shortage of supply, data from the Office for National Statistics shows that between 2005 and 2015 the city's average house price soared by 90% to £537,000 ($710,623). By contrast, the UK's average price rose 45% over the same period to £277,000.