Thousands of athletes and officials have begun arriving in London for the Olympic Games, as questions remain about recruitment of security staff, BBC News reported.

Preparations are intensifying 11 days before London 2012's opening ceremony.

The first priority "Games Lane" has begun operation on the M4 - the main route from Heathrow Airport - and the Olympic drug testing lab starts work.

Meanwhile, security firm G4S's chairman has refused to express support for his chief executive over the guards fiasco.

Speaking to BBC News channel on Monday morning, London mayor Boris Johnson said: "London is as ready, in fact readier than any Olympic city has ever been at this stage in proceedings."

Heathrow Airport is standing by to process as many as 120,000 passengers on Monday, about 10,000 more than would be normal for this time of year.

Immigration Minister Damian Green has said that the UK Border Force would be in full "Olympic mode" as of Sunday and he promised all immigration desks at Heathrow would be manned at peak times.

Volunteers will be directing athletes to the coaches and trains that will take many of them to the Olympic Village in Stratford, east London.

The village will house 16,000 athletes and officials at its peak.

The biggest anti-doping operation in the history of the Olympics also begins on Monday. Drug testers are expecting to take the first of about 6,000 samples for testing at the London 2012 laboratory.

Half of the competitors will be tested including every medallist at the Olympics and Paralympics.