One of London Mayor Boris Johnson's deputies resigned today over his misuse of a corporate credit card.

Ian Clement, deputy mayor for government and external relations, was reprimanded earlier this month for using the card to buy personal items in a breach of Greater London Authority (GLA) regulations.

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An initial investigation found that Mr Clement, the former leader of Bexley Council in south east London, had repaid the money at the earliest opportunity and had made no personal gain.

But further "discrepancies" were later discovered in the use of his corporate credit card and he stood down from the GLA today, City Hall said.

Mr Johnson wrote in a letter to his former deputy: "I was extremely angry at the liberties you took with your corporate credit card, but last week I made the essential distinction between behaviour that is crass and anything that amounts to dishonesty or deception.

"In the light of the further discrepancies in your expenses that have emerged this morning, it is clear to both of us that your position is untenable."

Mr Johnson initially stood by Mr Clement following press revelations last week that he spent £2,300 on personal expenditure using a City Hall credit card.

The deputy mayor's card was confiscated after he allegedly used it to upgrade himself and a colleague to business class on a flight to the Beijing Olympics, to pay for meals including a £700 dinner in New York and to buy more than £200 worth of groceries and a £535.49 audio system for his Jaguar, which he then repaid.

Mr Clement offered his resignation shortly after a conversation with Mr Johnson this morning in which it was made clear that he could not stay in the £127,000-a-year post.

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In a letter, he told the Mayor: "It has been a pleasure to work for you and I am sorry that I have let you down and deflected away from the excellent work you and indeed your team are doing in delivering for Londoners."

The Mayor's Office would not give details of the fresh allegations today.

But it is thought that a Greater London Authority investigation into Mr Clement's use of the credit card raised questions over occasions when it had been used on purportedly official business, such as lunches with MPs and councillors.

Mr Clement is the third deputy mayor to leave Mr Johnson's administration since his election in May last year, following youth leader Ray Lewis and transport chief Tim Parker.

City Hall said the post would be filled in due course.