Construction of London's 2012 Olympic Stadium is to start three months ahead of schedule, the Olympic Delivery Authority (ODA) said on Tuesday.

Work began on clearing the site last July and construction was originally scheduled to begin on the stadium by the time the Beijing Games start in August.

However, progress has been quicker than expected and, after the removal of vast quantities of contaminated soil, work on the bowl-like 80,000-seat stadium will begin in May.

It is a major boost to 2012 organisers, wary of the bad publicity over the Wembley Stadium project that was completed more than a year late.

"We have investigated the Olympic Stadium site, demolished tens of buildings and excavated 800,000 tonnes of soil," ODA chairman John Armitt said on Tuesday.

"Now that we have finished the work to dig the bowl where the opening and closing ceremony and athletics will take place in 2012, we are firmly on track to start construction work ahead of schedule."

The ODA said small amounts of low level radiological material have been found during excavations.

"This soil, with mild radioactive properties present due to the past industrial activity over the last 100 years or so, is being managed so that it presents no risk to public or workers," an ODA statement said.

"It has been stockpiled separately on site and will be taken away to be disposed of in the next few weeks."

The stadium, which will cost 496 million pounds, will be the focal point of the Olympic Park which is being built on former industrial land in east London.

The 2.5-square-km park will also house the Aquatics Centre, the Velodrome, the hockey, fencing, basketball and handball arenas and the international media centre.

After the Games the stadium will be scaled back to 25,000 seats. It will retain a permanent athletics track, although no long-term tenant has yet been found.