A man in overalls walks into the offices of a technology firm saying he is from BT and he needs to check the phone lines. The receptionist waves him through.

Minutes later, he walks out having collected passwords which will get him into the company's computer system.

Later he will steal the intellectual property on which the firm's future has been built and sell it to a competitor.

It sounds like thriller fodder - the plot of a cheap and implausible airport novel.

But this internet espionage is happening all the time, according to Worthing technology firm Pandora e-Security.

Computing intruders have reason to feel confident.

If they can break into NASA's computers and compromise the safety of British astronaut Michael Foale, as they did in 1997, they can walk through your defenses too.

In industry jargon, this sort of low-tech security breach is called social engineering.

Not only does it happen, if you pay chief executive Colin Dobson and managing director Ashley Hawes enough, they will make sure it happens to you.

Come to them with a security problem and the first thing they will do is a penetration test. They will break into your computers.

Or they will employ people to do so.

Mr Dobson cannot give the full CVs of these people, but they are "ex-Ministry of Defence security specialists" and they do not like being called "spooks".

Neither can he discuss his clients. Few firms advertise the fact they need security advice but his website says the firm's solutions secure the U.S. government, the FBI and the CIA.

Once the weaknesses in a organisation's computer defences are known, the two men make sure a myriad of evils - hackers, crackers and code kiddies - do not repeat the raid to steal or destroy vital information.

Social engineering is familiar to Pandora's clients and staff alike.

They have heard the story of a woman who phones IT departments in tears begging to be given the password to get into 'her' e-mail account late at night.

Or the many tales of computing managers who have revealed all over a few beers and a heated discussion about net security.

Many of the low-tech problems are the most devastating. Most hacking is internal and consists of nothing more complex than an employee tapping in a password after seeing it being used by their manager.

The experts call it shoulder surfing.

Internet security has never been far from the headlines in recent months.

When confidential e-mails to and from the Prime Minister found their way onto the front pages of the national Press, people asked whether someone had hacked Downing Street.

The credit card details of hundreds of customers of energy firm Powergen were recently revealed to a man poking around their site.

Even the biggest of governments with the most famous of security forces are worried.

Richard A Clarke, chairman of the U.S. Government's chief counter-terrorism group, said the U.S. could face an attack on the scale of an "electronic Pearl Harbour".

But it is not just vast American institutions which are vulnerable.

Many of the high-tech firms on Sussex's Silicon Coast own nothing but their knowledge and losing it would mean losing everything, Mr Dobson said.

"We have a globally based knowledge economy so intellectual property is the most important thing.

"If someone steals that, they can bring a firm down. Small and medium-sized businesses are being attacked more than anyone else."

Most people do not even realise they have been targeted.

Only when their competitors release products similar to their's or get to familiar customers quicker do they suspect a security breach might have taken place.

Companies do not need to be targeted directly, he said.

Small programmes called web crawlers are on the lookout for cracks in security which could give access to customer databases and other sensitive information.

"Within six seconds of a website being put up it is scanned," said Mr Dobson.

Now governments are moving in on the action.

Many Europeans fear the U.S. Echelon spy system is being used to eavesdrop on companies this side of the Atlantic to help American firms.

Mr Dobson claimed industrial espionage is such big business there is a league table of countries showing which spy most on businesses circulating in security circles.

It is, he said, a growth industry.

Bizarrely, Mr Dobson said, casual internet shoppers need not be worried by the boon in web spies.

The transfer of credit card details between buyer and shop is so straightforward and subject to so much security it remains as safe as handing over a card in a real store.

The communication and storage of business plans, research and development results and marketing schemes are far more open to prying eyes. So what can be done?

Naturally, Mr Dobson and his colleague said Pandora itself can help out small firms.

They work on a cost/risk analysis, which means they compare how much you have to lose with how much you are willing to pay to work out what it sensible.

There are novel technological toys to help keep computers secure.

A new Siemens mouse reads the fingerprints of its users.

But you could make a start by checking the credentials of the man from BT and remembering the tearful woman who has forgotten her password might not be telling the whole truth.