John McCain, the presumptive Republican presidential nominee, on Thursday denied having a romantic relationship with a female lobbyist and insisted he did not grant favours to the woman’s clients.

The rebuttal was in response to a New York Times story that detailed his close association with a telecommunications industry lobbyist as the Arizona senator prepared to make his first run for president in 2000.

The newspaper alleged that Vicki Iseman, whose clients included some of the biggest US cable and satellite television groups, accompanied Mr McCain to fundraising events and travelled with him on a client’s private jet.

The report offered no firm evidence of a romantic affair and no proof of special treatment for her clients. But it said senior advisers to Mr McCain confronted him and Ms Iseman about their close ties, amid concern that any appearance of impropriety could damage his presidential campaign.

“I’m very disappointed in this article. It’s not true,” said Mr McCain on Thursday, standing beside his wife, Cindy. “At no time have I ever done anything that would betray the public trust.”

Even the suggestion of unethical behaviour could prove damaging to Mr McCain because of his self-styled reputation as a crusader against political corruption – an issue he has put at the heart of his 2008 presidential campaign.

Both Mr McCain and Barack Obama, the Democratic frontrunner, portray themselves as political reformers who would challenge special interests as president, raising the possibility that ethics and integrity could be key issues in November’s election.

Charlie Black, a senior adviser to Mr McCain, dismissed the New York Times report as a “false smear campaign” by a liberal newspaper intent on undermining the almost certain Republican presidential nominee. “This doesn’t meet the journalistic standards of a thirdrate tabloid,” he told ABC’s Good Morning America, highlighting the story’s heavy use of unnamed sources.

In late 1999, Mr McCain twice wrote letters to the Federal Communications Commission on behalf of Florida-based Paxson Communications, one of Ms Iseman’s clients, urging quick consideration of the company’s bid for a television station licence in Pittsburgh.

Paxson’s then-chief executive was a large contributor to Mr McCain’s presidential campaign and the senator used the company’s private jet at least four times.

The McCain campaign said it was not unusual for the senator, who was then chairman of the Senate commerce committee, to write letters prodding regulators into action and dismissed the notion that they represented special favours. The letters to the FCC and Mr McCain’s ties to Paxson were reported at the time but his close relationship with Ms Iseman was not disclosed.

Both Mr McCain and Ms Iseman denied they were ever romantically involved. The senator said on Thursday that she was a friend. Mrs McCain said she trusted her husband, describing him as “a man of great character”.

Rumours that The New York Times was investigating Mr McCain’s ties to lobbyists had been circulating for months and his campaign fought to deter the newspaper from publishing the story.

Any perception of unfairness on the part of The New York Times could help galvanise support for Mr McCain among conservatives, many of whom view the newspaper as a symbol of liberal media bias. The McCain campaign on Thursday sent an e-mail to supporters asking for donations to help fight back against the “liberal media attack machine”.

But much of the focus on Thursday concerned the timing of the story. Critics of Mr McCain were suspicious that the New York Times, which endorsed him in the Republican race, might have delayed publication so as not to hurt his chances of winning the nomination.

Many conservatives mistrust Mr McCain because of his moderate stance on various issues, and believe his march towards the Republican nomination has been aided by sympathetic media coverage. The New York Times on Thursday said the story was published at the earliest opportunity after reporting was completed.

This is not the first time Mr McCain has faced suggestions of ethical impropriety. Shortly after entering the Senate two decades ago, he was accused of trying to influence banking regulators on behalf of Charles Keating, a financier later convicted of fraud.

The Senate ethics committee concluded that he had shown “poor judgment” but that his actions were not improper. Mr McCain has since become one of the Senate’s leading advocates for restrictions on political donations and lobbying.

Mike Huckabee, Mr McCain’s strongest remaining Republican challenger, said he accepted his opponent’s word that the story was false and added that the senator was a “good and honourable” man.