Angus Robertson, the Scottish National Party’s leader in Westminster, asked the prime minister why 3,250 department of work and pensions (DWP) staff have been assigned to investigate welfare fraud, while 300 specialise in dealing with the rich.

“Surely we should care equally about people abusing the tax system and those abusing the benefit system?” Robertson asked during prime minister’s questions. “Why has this government had ten times more staff dealing often with the poorest in society abusing benefits than with the super-rich evading their taxes?”

In fact the government confirmed on Wednesday that the ranks of DWP benefits investigators have swelled to 3,700 – a higher number than the one quoted by Robertson, and up from 2,600 in February last year.

That compares with 700 people who work at HM Revenue and Customs in the two units whose job it is to investigate the wealthiest 500,000 people living in the UK.

David Cameron was jeered when he admitted the figures cited by Robertson would need to be examined, but retorted “they sound to me entirely bogus”. He added: “The predominant job of the DWP is to make sure that people receive their benefits. The predominant job of HMRC is to make sure people pay their taxes”.

Benefits fraud costs the government £1.3bn a year, according to official statistics, while the gap between tax owed and tax paid is put at £34bn a year by officials.”