On 15 March 2015, the Conservative Party chairman, Grant Shapps, admitted that he had continued to work a second job as a “multimillion-dollar web marketer” under the pseudonym Michael Green after being elected to parliament in 2005 – despite previous denials.

The admission came less than two months before a general election, causing not inconsiderable political embarrassment for the government.

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Shapps, seen as something of a rising star in the Conservative Party, set up web sales business HowToCorp in 2005. He was elected as the Tory MP for Welwyn Hatfield at the 2005 general election.

The Conservative MP had consistently claimed that all his business activities had ceased the moment he became an MP. However, following a Guardian investigation, an audio recording from a conference in 2006 emerged in which Shapps could be heard boasting that his self-help products could make listeners a “ton of cash by Christmas”.

In 2012, pictures emerged showing Shapps at a $3,000-a-head US internet conference wearing a badge describing himself as “Michael Green”.

Faced with the Guardian story, Shapps conceded he had continued with his second job upon becoming an MP. The revelations triggered immediate calls from opposition parties for an investigation.

Lucy Powell, Labour’s election campaign vice chair, wrote to the prime minister, David Cameron, calling for a full investigation into whether Shapps had broken the ministerial code.

Powell wrote: “Such serious allegations cannot be allowed to hang over a man who has serious and wide-ranging ministerial responsibilities and who will be running your election campaign and so it is essential that this matter is immediately investigated…

“Mr Shapps’ repeated falsehoods demonstrate contempt for his constituents. Without clear answers from you today, you will also be guilty of demonstrating a disdain for decency and integrity in our public life.”

Powell also pointed to an LBC Radio interview, conducted three weeks before the Guardian story was published, in which Shapps said it was “absolutely clear” that he was not doing business as Michael Green while he was an MP.

The Tory chairman declared: “I thought the discussion here was second jobs whilst people are MPs. To be absolutely clear. I don’t have a second job. And I have never had a second job whilst being an MP. End of story.

“I wrote in a pen name, as many people have done in the past. The only difference is, I put my actual name and the pen name next to each other, which is why people know about it.

“It’s before I came into politics many years ago.”

Shapps acknowledged his error upon responding to the new allegations. He said he had “screwed up” the dates and “over-firmly denied” the allegations put to him in the LBC interview.

A Conservative Party spokesperson said: “Like many authors and journalists, Grant wrote with a pen name. This was completely transparent: his full name and biographical details were permanently published on the company’s main website.

“Given that this was a decade ago, and was mentioned during the cut and thrust of an interview, he referenced that his writing career had ended when he became an MP: in fact it ended shortly afterwards.”

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Despite the pressure, Shapps was publicly backed by Cameron and remained in his post for the duration of the election campaign.

Defending Shapps, the prime minister stated: “Grant did have another job when he first became an MP and he declared that in the register of members’ interests which is what you are meant to do.

“But he obviously made a mistake by saying in some interviews that the work had stopped earlier than it had. He’s put that right so I think we can put that behind him. He’s doing a good job.”