Thomas Casagranda, a former tax compliance officer with HM Revenue and Customs, said MPs’ tax affairs were effectively “above the law” because senior managers refused to scrutinise them.

Mr Casagranda, who quit his post at HMRC’s Reading office 18 months ago, said he had been “shocked” by their reluctance to act.

He confirmed he suggested an investigation after receiving a tip-off that an MP was failing to declare rental income from a property.

He said: “During a routine meeting to discuss possible projects, such as investigating taxi drivers or subcontractors in the building in­­d­­­u­­­stry, I suggested running a project on MPs.

“The meeting went on as normal but afterwards I was taken aside by the head of the regional risk intelligence team and told I would be ‘out on my ear’ if I mentioned MPs again. I was told that if I looked at an MP’s tax return or checked them out on PAYE records I would face disciplinary proceedings.