Ken Livingstone received a secret donation from a property developer with a conviction for fraud after the London mayor championed the businessman’s plan for a 46-storey skyscraper.

Gerald Ronson, the tycoon jailed for his role in the Guinness share-dealing scandal, wrote a cheque to Livingstone for £4,990 – just £10 below the £5,000 threshold at which donations have to be publicly registered with the Electoral Commission. The disclosure raises questions over Livingstone’s use of a legal “loophole” which has allowed him to keep secret the identities of campaign donors.

A spokesman for Livingstone, who is seeking a third term as mayor in May’s London elections, last night refused to reveal whether there were any other similar hidden donations.

Ronson, 68, made the payment to the Labour mayor’s re-election campaign two years after he received planning permission for the Heron Tower development.

The 202-metre skyscraper was opposed by English Heritage and the dean of St Paul’s Cathedral because they said it would damage the capital’s historic skyline. However, Livingstone said it would “support London’s economic future”.

The Ronson donation was made shortly before Livingstone won the June 2004 London mayoral election. A spokeswoman for the developer said: “I can confirm it was a personal cheque from Gerald Ronson. The exact amount was £4,990.”

The Conservatives claimed that Livingstone was exploiting a “loophole” in election law which allows him to hide cash gifts by maintaining they are payments to the Labour party rather than his personal election fund.

If payments are made to a political party, only those above £5,000 have to be declared to the Electoral Commission. However, the threshold for individual politicians – the so-called “regulated donees” – is much lower at just £1,000.

Boris Johnson, the Tory mayoral candidate, declares every donation to his campaign over £1,000. But Livingstone has never made any declaration as a “regulated donee”.

Greg Hands, the London Conservative MP, said: “With just over a month to go before before the election, Mr Livingstone must come clean about the size of his donations and the identity of his donors. The Labour mayor is shamelessly exploiting loopholes in the law to hide controversial payments.”

Livingstone has approved a series of skyscraper projects for the capital. “I have no objection in principle to London having the tallest of buildings,” he said.

In 2001 Ronson’s Heron Tower development was approved by the Corporation of London. Livingstone’s submission backed the giant office block. Amid the protests John Prescott, the deputy prime minister, called in the project for a full public inquiry – a decision that was publicly opposed by Livingstone at the time.

In July 2002 Prescott announced that he would, after all, grant planning permission to Heron Tower. Livingstone put out a press release praising the development and claiming it would “keep London competitive”.

The 2004 cheque is understood to be the only cash donation that Ronson has made to Livingstone. In April 2006 Ronson also gave him a pair of opera tickets worth £160.

Ronson served six months in jail of a year-long sentence for his role as one of the so-called “Guinness Four”. He has since rebuilt his business empire and reputation.

The London Labour party said: “The Labour party receives all donations and these are declared to the Electoral Commission in accordance with the rules governing donations to political parties in the usual way.”

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