Athletes arriving for the London 2012 Games are being told to keep out of the main Olympic Park because it is not yet ready.

With less than two weeks to go to the opening ceremony, the main park remains a construction site, and national teams, who begin arriving in London tomorrow, will have to stay away.

One team chief said yesterday: “We were told the Olympic Park won’t open until the 23rd. This is a week later than what we expected. We were told this is because of some construction issues in the park.

“We were assured they are relatively minor but it has come as a surprise because we had been told for some months now that the park was on time and ready, and it isn’t.’’

The disclosure came after a series of blows to organisers last week over the state of preparations, the most damaging being that troops are being called in after G4S, a private security firm, admitted it could not provide enough guards.

Yesterday its chief executive apologised for the fiasco, which will cost the firm up to £50 million, but admitted that he did not know if his security staff could even speak English.

About 30 teams, comprising about 1,000 athletes, are expected to take up residence in the athletes’ village, next to the Olympic Park, tomorrow with 5,000 expected by the end of the week.

More than 40 team bosses met on Friday at the athletes’ village and some were clearly taken by surprise by the admission that certain areas of the park would remain off limits. The Olympic Park, which hosts athletics, swimming, hockey, cycling and other sports, is still officially a construction site. Some contractors, who had passes valid up until Friday, are having to redo biometric tests to be able to gain access.

Venues, however, are finished, and athletes’ training programmes should not be disrupted.

Sources said the problems had been caused by the wet weather and security issues, but Locog, the London organising committee, said that work in the park was on schedule.

By Jacquelin Magnay and Robert Mendick