Documents here revealed Monday a massive operation by UK security chiefs to tighten access to every venue ahead of the London Olympics. A terrorist sleeper cell could be already in place and waiting to commit an Olympics atrocity, experts fear. Security chiefs have identified existing staff as the greatest threat to safety and told workers to watch each other for suspicious behaviour and to scrutinise all deliveries to Olympic venues, the Daily Mirror newspaper said. A source told the paper \"There are real concerns that a terrorist cell could be lying under the radar already. \"Deliveries to venues have been identified as the main threat, because of the access it gives. \"The worry is that someone who is already employed is just waiting for the opportunity to attack.\" Last week, the domestic intelligence service, MI5, director general Jonathan Evans warned that events outside London could be the prime targets. Fans will descend on locations such as Weymouth for sailing, Eton Dorney for rowing and Old Trafford in Manchester and Other grounds in Newcastle, Coventry, Cardiff and Glasgow for football. Under Games security procedures, security-compliant certified goods can be delivered direct to Olympic venues. But uncertified goods must be screened, then loaded on to pre-approved vehicles and delivered only by authorised drivers. The source added: \"Bosses are determined not to make a single slip-up in their preparations. \"There will be no risks taken with the safety of athletes and spectators when the eyes of the world are on London.\" Checkpoints will control access to all venues and even goods for Westfield Shopping Centre - near the Olympic Stadium - will be screened before getting in. The source added: \"Shops are preparing for huge disruption, but they know there is no choice.\" Intelligence gathering has identified Irish dissidents, Islamist extremists and \"lone wolf\" attackers as the main threats. More than 500 MI5 officers are working solely on Games security, the newspaper said. The Olympics are due from July 27 until August 12.