Marcial Boo, the chief executive of Ipsa, has now disclosed that the data was available for four hours and 50 minutes and as accessed "by around three dozen people". He issued an appeal to those who downloaded the information to ensure it is "permanently deleted or destroyed".

He said that "appropriate disciplinary action" will be taken against those who put the files online, but did not say if anyone would be sacked.

MPs who saw the data told The Telegraph that the data had the potential to be "hugely embarassing" for MPs who employ their wives, children and other "connected parties".

According to official records a total of 150 MPs - equivalent to nearly one in four - employ a "connected party", at a total cost to the taxpayer of £3.6 million. The Independent Parliamentary Standards Authority (Ipsa) only publishes anonymised information about salaries and bonuses paid to staff.