British airport operator BAA admitted on Monday that immigration queues at Heathrow Airport, the main gateway for the London Olympics, have been "unacceptably long" in recent days.
Some passengers were forced to wait for two and a half hours last week for passport checks at the west London airport, which will welcome around 80 percent of the athletes, officials, fans and media coming for the Games.
"Immigration waiting times for passengers during peak periods at Heathrow in the last few days have been unacceptably long," a BAA spokesman said.
Hours-long immigration queues at the airport, the world's busiest in terms of international passenger traffic, caused major embarrassment to the British government in May, just weeks before the Olympic opening ceremony on 27 July.
With thousands of athletes due to land at Heathrow over the next three weeks, there are fears that the Home Office, or interior ministry - which is responsible for immigration - will not be able to cope with the traffic.
The Daily Telegraph newspaper reported half-mile queues at Heathrow's Terminal 4 on Friday.
A Border Force spokeswoman said the recent "queue breaches" at Heathrow had been "less than an hour" and that extra staff had been sent to immigration desks to deal with them.
"We are fully prepared for the busy Olympic period and will be implementing our well-rehearsed plans," she said.
"This includes staffing all immigration desks at key ports whenever necessary during the peak Olympic and Paralympic arrivals period."
BAA said the Home Office had promised that all of its Border Force desks at Heathrow would be manned during the busiest Olympics arrival times.
"Immigration is a matter for the Home Office," the BAA spokesman said. "The Home Office has said that from July 15 all Border Force desks at Heathrow will be open during peak arrival periods."
The Border Force aims for passengers from outside the European Union to pass through passport controls in under 45 minutes for 95 percent of the time, while the target for EU passport holders is under 25 minutes 95 percent of the time.