LONDON - London's mayor warned Monday that long delays for passengers arriving at the city's largest airport risk damaging Britain's reputation in the run-up to this summer's Olympic Games.

Boris Johnson wrote to Home Secretary Theresa May amid increasing concerns over lengthy lines at border control desks for those arriving at Heathrow Airport.

Passengers have complained of long delays, on some occasions of more than an hour, because of lengthy passport and visa checks and a lack of border staff.

Johnson said the delays were giving "a terrible impression of the U.K." and demanded action ahead of a busy summer during which Britain will mark Queen Elizabeth II's 60th anniversary as monarch in June, and hold the Olympics.

"This summer, with the Queen's Diamond Jubilee and the London Games, gives us a once in generation's opportunity to promote what is very best about London and the U.K.," Johnson wrote.

"It is critical for the success of these events that visitors from across the globe have a great experience from beginning to the end of their stay," he told May, who is responsible for Britain's borders.

Heathrow typically handles an average of 190,000 arriving and departing passengers each day, but is braced for a major influx during the July 27-August 12 Olympics.

Johnson said that about 80 percent of visitors and participants arriving for the Olympics will use Heathrow, which would "place short term additional pressures on the system."

Lawmakers and Heathrow's operator BAA Ltd. have called for additional border staff, mindful of the disruption experienced last summer, when Britain's then border chief relaxed some passport checks during the busy tourist season simply to handle the demand.

In a statement issued late Sunday, Britain's Border Force — which runs airport checks — and BAA Ltd. insisted problems were not widespread.

"The majority of passengers pass through immigration control quickly but there are sometimes delays at airports for a range of reasons," the statement said. "We will not compromise border security but we will work together to keep delays to a minimum."