Although, the Nationwide added, house prices remain £11,000 higher than three years ago, last month's 1.7 per cent fall in values is double the rate of decline for May and suggests the property slump is accelerating. HBOS warned of a 20 per cent fall in values through 2008 and 2009 and revealed the number of its customers in mortgage arrears has risen from 35,600 at the start of the year to 39,300.