It is just the latest in a string of computer problems that have plagued Nats
in recent years at its headquarters in Swanwick, Hants.

A consultant who has worked for Nats said it knew its software needed to be
replaced a decade ago but will be relying on the 1960s programmes for
another two years.

Martyn Thomas, Visiting Professor of Software Engineering at the University of
Oxford, said: “The National Airspace System that performs flight data
processing was originally written for American airspace in the late 1960s.

“It wasn’t designed to cope with the volume of air traffic we have today, or
to interface with modern computer software.”

Prof Thomas said the NAS system was written using a now defunct computer
language called Jovial, meaning Nats has to train programmers in Jovial just
to maintain the antiquated software.

He said: “When Swanwick opened in 2002 there were a number of failures because
as air traffic grew NAS wasn’t able to handle the additional complexity and
the extra information they were having to put in.

“Some of the other software they use dates back to the 1980s, and they knew a
decade ago that it needed replacing, but they haven’t made it a priority.

“Britain is now the only country that is using the 1960s software. It is due
to be replaced with a new system from a Spanish company in about two years,
but until then they will just have to manage.”

Heathrow, the country’s busiest airport, had to cancel 50 flights, with around
20 more scrapped at Stansted and dozens more delayed or diverted to airports
in Europe from Gatwick, London City and Luton. Delays were also reported at
Birmingham, Manchester, Bristol, Aberdeen and Edinburgh.

Passengers left stranded included Nigel Farage, the Ukip leader, who was on
his way to Vilnius in Lithuania to make a speech when his flight was delayed
at Stansted Airport in Essex.

He said: “I don’t know what is going on – there’s a lack of information.
Economically it is a catastrophe for the country.

« We are an international trading nation, and this is bad. It’s really not
good. »

It was the second major computer failure in 12 months at the £700 million Nats
facility, which opened five years late in 2002 and cost more than five times
the original budget.

Last December another computer failure meant Nats had to reduce flights by 20
per cent on a busy Saturday, leading to major disruption.

The Swanwick centre controls all flights south of Manchester, meaning incoming
flights were grounded at most European airports and aircraft already in the
air had to land in other countries. Alastair Campbell, the former Downing
Street communications secretary, was among passengers who unexpectedly found
themselves in Paris because of London airspace being closed.

The computer error lasted from 3.27pm to 4.03pm, and Nats said that while it
had not been caused by a power cut, it was too early to say exactly which
system failed.

A spokesman said: “As with any safety-critical industry, we will have to have
a full investigation to determine the exact cause.

“We need to see if there are any other systems or underlying root causes that
haven’t been addressed today, so we cannot speculate on whether it might
happen again.”

A spokesman for Heathrow Airport said: « The earlier problem will cause
delays and cancellations to flights for the rest of the day and is likely to
have a knock-on effect on some services tomorrow because aircraft and crew
will now be out of position.

« Passengers due to depart today and tomorrow should check the status of
their flight with their airline before travelling to Heathrow. We are very
sorry for the disruption to passengers’ journeys. We have extra staff on
duty to help passengers. »