Questions have been raised about security at the Olympic Park in east London after a worker managed to smuggle a fake semtex bomb on to the site.
According to The Sun On Sunday newspaper, the unidentified man drove past two checkpoints in a digger without being challenged.
The 'device' was contained in a clear plastic container placed inside the cab.
The security breach occured just hours before the Olympic stadium was officially opened ahead of an athletics test event attended by 40,000 members of the public.
The Sun says it gave the man the dummy bomb after he contacted them to express his concern about apparent security loopholes at the Park.
He said it worried him that he was only searched when he first arrived for work each morning - and never again.
Video shows him driving around the Olympic Park without once being stopped by security guards.
During a 25-minute tour he took photos of the bomb near the main stadium, the Velodrome and the athletes village, which will house some 17,000 competitors during the Games.
The builder told the paper: "I've worked for several years on the Olympic Park and keep hearing how the security is water-tight.
"But after a thorough check every morning, I can come and go from the Park all the day with my digger without being searched at all.
"I can meet anyone on the outside without anyone knowing and bring anything inside without anyone checking. If I had terrorist connections I could be bringing in explosives, chemicals - anything at all.
"It's a massive security loophole."
A Home Office spokesperson said Games organisers Locog were responsible for security at the Olympic Park they had been asked to "look into this incident and report back to the Home Secretary urgently".
A statement from Locog said: "Clearly a tupperware box containing batteries, a mobile phone and some loose wires would not pose a security threat on a construction site.
"However, we will be looking into the allegations to ensure our security regimes are as robust as they need to be at this time.
"As you would expect our security increases significantly the closer we get to the Games. The Park and the Village will be searched and sealed before it is locked down for Games time."
Around £1bn is being spent on security at the Olympic Park. There are 23,700 guards, who will be boosted by 14,000 troops at key times.
The park its itself is surrounded by an 11-mile electric fence costing £80m.