Olympic venue Dorney Lake is closed to the public from today.
Authorised people only are allowed on the site now as preparations for London 2012 rowing and canoe sprint competitions reach their climax.
Security measures will be stepped up further after the Olympic torch visits the venue on July 10, and the first day of rowing will take place on July 28.
Ivor Lloyd, the venue's managing director, said: "We're now in the first stage of lockdown and there is no access for members of the public.
"After the torch relay we'll be in final lockdown and everybody's going to be treated by security as if they're at the airport going on holiday."
About 30,000 people will visit the venue off Court Lane for the Olympics from July 28 to August 11 and the Paralympics from August 31 to September 3.
Spectator areas are '80 per cent completed', with two grandstands being built to the north of the lake and another to the south for VIPs, media and athletes' friends and relatives.
Ivor said: "We'll be open for business again from September 12 but it isn't yet known when the site will be opened up to members of the public.
"We're anticipating we'll have at least six to nine months of reinstatement to manage in order to put the venue back as it should be."
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