Balfour Beatty pleaded guilty last year to failing to ensure the safety of its employees and the public in the collapse during the construction of the Heathrow express rail link in September 1994. The tunnel was empty and no one was killed or injured.
A judge at the Old Bailey, Mr Justice Cresswell, said yesterday: "This was one of the worst civil engineering disasters in the United Kingdom in the last quarter of a century.
"The tunnels were being built below part of the world's busiest international airport and there was considerable potential for harm. It is a matter of chance whether death or any serious injury resulted from those very serious breaches."
Hundreds of flights were cancelled when an enormous crater appeared in between the airport's two main runways, dragging down car parks and causing buildings to sway.
In a statement, Balfour Beatty said it "deeply regretted" the collapse and said working practices across the group had been reviewed after an investigation.
Another firm involved, Geoconsult, was found guilty of two charges brought under the Health and Safety at Work Act 1974, of failing to ensure the safety of employees and the public. The Salzburg firm, which denied the charges, was fined pounds 500,000. Geoconsult was monitoring the rail link during the four-month period from May 1994 when the tunnel was being built by Balfour Beatty. It was responsible for the design and the technical supervision of the tunnel.
The companies were ordered to pay costs of pounds 100,000 each.