Long queues at immigration at Heathrow Airport give a "terrible impression" of Britain as it seeks to attract tourism and investment, Mayor of London Boris Johnson has said.
There has been increasing anger over the delays faced by passengers arriving at the airport, with queues up to three hours long reported.
Mr Johnson , who is campaigning for re-election on Thursday, has written to Ms May about his "serious concerns" about the impact of the problems during the London Olympics, when passenger numbers are expected to soar.
"Heathrow is the main gateway for Londoners returning home and foreign visitors arriving in London and the UK to do business or travel and is often their first experience of both our capital and our country," he said.
"It is extremely unfortunate that, in a time of economic difficulties, when I as Mayor and the Government are working so hard to attract inward investment to London and the UK, that our main port of entry is gaining such a poor reputation."
Nur Alam of the PCS union, which represents Border Force staff at Heathrow , told Sky News the queues were caused by a lack of staff.
"I think BAA are doing the best they can given the situation. It's definitely affecting travel into the UK and will affect tourism," she said.
Blaming Government cuts to the Border Agency, she said: "I don't think they are putting due importance on the primary checkpoint."
The Government will make an emergency statement in Parliament on the issue after the speaker granted an urgent question to Commons Home Affairs Committee chairman Keith Vaz.
Earlier, a spokesman for the Prime Minister said: "We have to ensure we do not compromise security but equally ensure a regular flow of passengers and keep disruption to a minimum.
"We have a contingency force of appropriately trained staff who will be sent to the border during the Olympics."
Meanwhile, border officials reportedly stopped Heathrow from handing out leaflets detailing how passengers could complain to the Home Office .
According to the Daily Telegraph, the director of UK Border Agency operations at Heathrow Marc Owen told airport operator BAA that the leaflets were "inappropriate" and that ministers would take "a very dim view".
Mr Owen also reportedly told BAA to prevent passengers taking pictures in the arrivals hall.
Former transport minister Jim Fitzpatrick, Labour's aviation spokesman, described the move as "a pure cover-up".
"This is further evidence of Border Force trying to hide the severity of the problem," he said.
Boris Johnson is one of seven candidates for the Mayoral elections on May 3.
The Mayoral candidates are:
Cortiglia Carlos - British National Party
Johnson Boris - The Conservative Party Candidate
Jones Jenny - Green Party
Livingstone Ken - The Labour Party Candidate
Paddick Brian - London Liberal Democrats
Webb Lawrence James - Fresh Choice for London