Watford MP Richard Harrington has defended Conservative Party chairman Grant Shapps in the MP second jobs row – calling it a “technical error”.

Mr Shapps said he “screwed up” in an interview with LBC Radio in which he “over firmly” denied working as an a web marketer under the pen name of Michael Green after being elected in 2005.

But after The Guardian issued recordings of Mr Shapps making a sales pitch in 2006, Mr Shapps said he was “mistaken” on the dates.

Mr Harrington, the Tory vice chairman, has backed Mr Shapps today and said calls for his resignation were “ridiculous” after Labour called for an inquiry into whether Mr Shapps had breached MP rules.

The Watford MP said: “He had declared it on the registered members’ interests but the important thing is this was a decade ago.

“He was asked ten years later about the dates. I don’t make excuses for that, but it was a technical error and not a deliberate one.”

Mr Shapps used to run the HowToCorp website which featured get-rich-quick advice from supposedly successful businessmen Sebastian Fox and Michael Green.

His register entries between 2005 and 2007 stated that he had a remunerated directorship and a registrable shareholding in How To Corp Ltd.

Mr Shapps last month denied that he had continued to work as an internet marketer after being elected in 2005.

He told LBC radio: "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."

But the recording obtained by The Guardian captures the MP in 2006 selling business self-help guide Stinking Rich 3 and claiming his products could make listeners a "ton of cash by Christmas".

Mr Shapps initially dismissed the issue as an "old story", but the Tories said that although the chairman had talked of his writing career having ended when he became an MP, "in fact it ended shortly afterwards".

The BBC later reported Mr Shapps told a reporter he had "screwed up" on dates and stated his case "over-firmly" in the LBC interview.

Mr Harrington said he believed MPs should not have second jobs.

He said: “I believe being an MP is a six-day-a-week occupation, any examination of my diary would show this.

“That doesn’t mean the odd meeting elsewhere is not possible but anything beyond that is incompatible with the tasks I have to do.”

Labour’s Parliamentary candidate for Watford, Matt Turmaine, has called for an end to MPs taking second salaries – including from directorships and consultancies.

He has pledged to work solely as the town’s MP if elected in May.

Mr Turmaine said: “We have all been shocked again at the efforts some MPs make to boost their income when they should be concentrating on being an MP.

“I call on the other parliamentary candidates for the general election in Watford to join me in supporting a ban on second jobs for MPs.

“If I’m elected in May, I won’t be doing any second jobs – can the other candidates say the same?”

Liberal Democrat candidate Dorothy Thornhill has said she will remain Watford's Mayor for a year if she were to be elected.