The main Olympic Stadium in Athens could be left without its protective roof after a fresh construction delay, The Times, London, reported.

The International Olympic Committee issued a final warning to the Greek government, giving it a deadline of May 20 to complete the frame of the roof of the main stadium.

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If it fails to do so, all construction will be halted and the stadium will be without a roof, leaving athletes and 55,000 spectators to swelter in the August heat.

The roof was meant to be Greece's greatest architectural triumph since the Parthenon, but it has been beset by construction delays and has repeatedly missed IOC deadlines. There are fears in Athens that it could become a symbol of Greek failures.

The main stadium's revolutionary 18,000 ton steel-and-glass roof, designed by Spanish architect Santiago Calatrava, resembles the ribcage of a whale. It incorporates two giant arches that need to be locked together in a complex sliding operation. Calatrava had envisioned the structure as the signature image of the Athens Games and called it his Olympic dream.

Each half of the roof has a large bow arch supporting a shallower spine beam, which in turn supports the ribs that cantilever out to form the canopy.

The statistics are awe-inspiring. Both halves of the roof are more than 300m long and the 80m high bow arch is made of tubular steel 3.6m in diameter.

According to Max Jones, performance director for UK Athletics and chief coach for the British Olympic team, "The main problem is for the spectators. If it gets really hot and there is no roof, it will be unbearable for them. It would also be better for the athletes if the stadium had a roof. Then they could find some shade. If it is 40ºC it will not be a pleasant experience."

The Greek authorities are already facing criticism over construction delays and security arrangements for the Games.

An IOC spokesman said, "May 20 is the cut-off date. If by then the slide has not been done, we would have to say that work should stop and we must apply contingency plans."

Plans to build a roof over the swimming pool in Athens have already been shelved.