The International Olympic Committee has confirmed today that Athens will be ready to host this summer's Olympic Games, which are due to begin on August 13.

Denis Oswald, the chief IOC supervisor, said that the organisers had managed to overcome the worrying delays after he had conducted a major three-day inspection, the last before the Games are due to begin.

He said: "In the past we had doubts. I am very happy to report all these doubts have disappeared." The upbeat tone was in marked contrast to previous visits, when Mr Oswald and others pressed for faster work and more substantial results.

The green light came as a huge relief to Greek Olympic planners who have faced ceaseless criticism and questions about their ability to put on a successful Olympics.

The IOC was so worried about delays and bureaucratic inertia four years ago that it said the Games could be in jeopardy. But today Mr Oswald said: "No single project is at risk and we know everything will be delivered on time."

Gianna Angelopoulos-Daskalaki, the chief organiser, summed up the mood: "Thank heaven that this progress is finally being made."

Greece is the smallest nation to host the summer Olympics since Finland in 1952 – a time when the scope of the Games and security issues were far less daunting.

"Seven years ago we promised that Athens will be good for athletes and Greece would be good for the games," Ms Angelopoulos-Daskalaki said. "Our promise is being kept."

Mr Oswald added that the round-the-clock efforts to install the massive roof over the main Olympic stadium stood as a "sign of the capacity of Greece" to meet the tight deadlines.

Engineers on Monday began moving one of the 8,000-tonne arches along specially built tracks. The glass and steel roof is designed to shade the 75,000-seat stadium from the intense summer sun.

The work eased IOC worries about whether the stadium could be completed by the end of June. Organisers also hope the web of metal and glasslike strands will be an enduring image of the Athens Games.

"We have very good news about construction and, of course, the most symbolic is the sliding of the roof," Mr Oswald said.

Mr Oswald did strike a note of caution however, by saying that time was limited to conduct all the proper pre-Olympic checks on the venues.

The issue of security also remains a concern and Mr Oswald has met with Greek authorities and representatives of a seven-nation security advisory task force, which includes the United States and Britain.

Mr Oswald said the triple bombing outside an Athens police station last week "did not affect" the IOC's view of the security planning, which is costing Greece at least €1 billion (£674 million). There have been no arrests for the attacks and no claim of responsibility.

Although no more full IOC inspection visits are planned, Mr Oswald said he will make follow-up stops in Athens before the Games commence.