Rudy Giuliani denied any wrongdoing today over claims that trysts with his mistress while he was Mayor of New York cost taxpayers tens of thousands of dollars — and that he tried to cover up the expenses.

According to records obtained by a respected US political website, Mr Giuliani billed New York City for thousands of dollars in expenses for his security detail, which accompanied him on trips to Long Island while he visited his mistress.

Many of the security expenses were invoiced to obscure city agencies, such as the New York City Loft Board, giving the impression somebody did not want the expense claims to be linked to Mr Giuliani. The expense receipts tally the cost of hotel and petrol bills for police detectives who travelled everywhere with Mr Giuliani, according to the website Politico.com.

The Republican presidential frontrunner's record as New York mayor is already facing closer scrutiny after the indictment this month of his close friend Bernard Kerik, whom Mr Giuliani appointed as the city's police chief.

Many of the bills were from hotels and petrol stations on Long Island, where Mr Giuliani began visiting Judith Nathan, now his third wife, in the summer of 1999.

Politico reports 11 Long Island trips indicated by credit card receipts. It is not possible to say how many of those visits included official business, but eight of them were not noted on Mr Giuliani's mayoral schedule.

The first trip to Southampton, Long Island, where Ms Nathan had a flat, appear in travel documents running from August 31 to September 1, 1999. Four police officers spent the night in a nearby motel, billing the city $1,016.20 (£485). Mr Giuliani's schedule listed no events in Long Island that day.

Asked about the expenses at a Republican debate last night, Mr Giuliani denied any wrongdoing. He said that New York mayors were given round-the-clock security, and that he knew nothing about the eexpense claims.

“I had nothing to do with the handling of their records,” Mr Giuliani said. “They were handled, as far as I know, perfectly appropriately.”

A Giuliani adviser, Anthony Carbonetti, described as “nonsense” any suggestion that Mr Giuliani had sought to hide anything. “There was no effort to hide anything — on the mayor's part definitely, because this is stuff he has never seen,” Mr Carbonetti said.

He said that he had ordered an investigation into why the expenses were billed to the obscure New York city agencies.

Mr Giuliani received 24-hour police protection when mayor and there is no suggestion that he used that detail improperly.

Receipts show that Mr Giuliani was in Southampton every weekend in August and the first weekend of September 2001, just before the September 11 terror attacks.

Questions about the expenses were first raised by city auditors late in 2001, when an audit found $34,000 of travel expenses in the accounts of the Loft Board. When asked for an explanation, Mr Giulaini's aides refused, citing “security”.

In 2002, New York auditors found $10,054 in security expenses billed to another unlikely agency: the Office for People With Disabilities. Another $29,757 was billed to the Procurement Policy Board, according to the documents obtained by Politico.com.

Mr Kerik was indicted on November 9, 2007, on fraud and corruption charges. He surrendered to the FBI after a grand jury indicted him on 16 counts of tax fraud and corruption, which carry a maximum sentence of 147 years. Many of the charges stem from when Mr Kerik was New York police commissioner.

Mr Giuliani is also godfather to two of Mr Kerik's children. Mr Kerik had been under investigation for 18 months.

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