Vince Cable faces possible suspension from the House of Commons, after failing to declare opinion polls commissioned for him by his ally Matthew Oakeshott, in his register of interests.
The parliamentary standards commissioner today confirmed to Politics.co.uk that they had launched a formal inquiry into the business secretary.
"We received a complaint into Doctor Cable, and have decided to begin an inquiry into his registration of interests," a spokesperson said.
The inquiry was launched after the blog Political Scrapbook complained that Cable and his aide Tessa Munt, had failed to declare the polls in their parliamentary register of interests.
In an interview with the BBC last month, Vince Cable admitted that he was aware the polls had been commissioned for him.
"In this particular case, Lord Oakeshott asked my election campaign manager if we wanted a poll done in my constituency. We said ‘yes’. It was a private local poll for general election planning and obviously nothing to do with national leadership”
The polls, conducted by ICM would have cost tens of thousands of pounds to conduct.
The standards commissioner will now investigate whether Cable and Munt should have declared them in their register of interests.
If found to have breached parliamentary rules, Cable could face further sanctions from the Committee on Standards and Privileges.
Sanctions include being forced to apologise to the House and in extreme cases, suspension or expulsion.
The investigation is the latest embarrassment for the business secretary, after his close ally Lord Oakeshott was revealed as being behind an apparent plot to oust Nick Clegg.
The Lib Dem leader has previously said that he believes Cable's claims not to have been involved in any plot.