BBC Transport Correspondent Simon Montague: The collapse could have caused a major disaster

The tunnel collapsed in the early hours of October 21, 1994. Fortunately no-one was in the tunnel at the time.

However, the court was told that the collapse could have crushed to death people using the nearby Piccadilly tube line and the judge hearing the case, Justice Peter Cresswell, said it was "luck more than judgment" that this did not happen.

Huge crater

The collapse could have ended in real disaster

The engineering disaster caused a huge crater to appear between the airport's two main runways and caused damage to car parks and buildings. It took months to clear up the damage.

The collapse occurred during the construction of a tunnel for the Heathrow Express Rail Link.

Balfour Beatty pleaded guilty last year to failing to ensure the safety of both its employees and members of the public.

Austrian engineering firm Geoconsult, which was responsible for monitoring the progress of the Heathrow Express Link, was also fined £500,000.

The judge ordered both companies to pay a further £100,00 each in costs.

Terrible disaster

Justice Cresswell said: "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."

Both firms were found guilty of falling "seriously short" of the appropriate standards and reasonable steps, resulting in serious breaches of the Health and Safety Regulations.

Martin Thurgood, of the Health and Safety Executive, told the BBC: "This level of fine sends a clear message to the industry, whether (to) the client's designers, contractors or sub-contractors, that they do need to take full account of workers safety and public safety in the way in which they go about their business."

Balfour Beatty, based in Edinburgh, is one of the UK's largest building groups, and is part of the engineering group BICC. It said it deeply regretted the accident.