Henley MP Boris Johnson could face a Parliamentary inquiry for failing to declare shares in a TV company - just days before the London mayoral election.

The Tory candidate, who has made much of his plans for greater financial transparency in City Hall ahead of next week's vote, failed to list on the register of members' interests his shareholdings in a company making documentaries he presented.

Labour seized on the omission, reported in the Evening Standard newspaper, as evidence of Mr Johnson's "sheer ineptitude" and said it called into question his competence to run the capital.

But Mr Johnson, who owns one third of the company, Finland Station, said he had not been aware of rules requiring MPs to register any shareholding greater than 15 per cent.

Labour MP Karen Buck, who has written to the Parliamentary Commissioner for Standards to seek an investigation into the matter, said: "Boris Johnson is trying to repackage himself as a serious and competent politician whilst claiming that his failure to comply with rules on reporting these share holdings to the Parliamentary commission was simply an oversight.

A spokeswoman for Mr Johnson said: "Boris Johnson has declared a payment from Finland Station of £30,000 as writer and presenter of a documentary.

"This is the only income he has received from the company."

Mr Johnson said he would stand down "very soon" in the event of toppling current mayor Ken Livingstone on May 1.

His resignation would trigger a by-election, with Henley residents asked to choose their MP for the second time in three years.