It remains a golden rule for spies of any persuasion that a walk in the park is still the safest environment for receiving secret information verbally from an agent.

Anything divulged inside a building or a private car is potentially open to an extraordinary array of eletronic bugging devices or telephone intercept systems. Bugging is a fine art, and the technology has leapt forward in recent years.

The electronic bug allegedly used by the police to eavesdrop on the conversation between Babar Ahmad, suspected of having links to terrorist organisations, and Sadiq Khan, his constituency MP, during a meeting in Woodhill prison in Milton Keynes, was probably the conventional type.

The basic form of bug requires someone to listen in from several hundreds yards away, or to have a recording system hidden nearby that can store many hours of conversation.

However, the latest electronic listening device is known as the GSM bug. Michael Marks of Spymaster, a company that supplies surveillance equipment, told The Times: “With one of these new bugs, all you have to do is place it covertly under someone’s desk. It’s like a miniature cellular phone. You can ring it from thousands of miles away, it answers silently and you can listen in on conversations. The GSM bug could be in an office in London but the person listening to the conversations could be in Australia.”

The renegade former MI5 officer, Peter Wright, author of the controversial Spycatcher memoir, claimed that in his early years as a Security Service officer, “We bugged and burgled our way across London at the state’s behest, while pompous bowler-hatted civil servants in Whitehall pretended to look the other way.”

Since those cavalier days in the 1950s and 1960s, bugging and burgling on behalf of the state have had to be authorised with warrants. Neither ministers nor senior civil servants have been allowed to turn a blind eye, because the security and intelligence services are accountable to British law and Parliament.

However, the level of authorisation varies according to the operation. Interception of telephone calls, post and e-mails has to be authorised with a warrant from a Secretary of State, normally the Home Secretary or Foreign Secretary, but also the Northern Ireland Secretary in dealing with Irish terrorism.

There are also two types of covert surveillance - directed and intrusive. The latter where police officers or MI5 officers seek to place electronic bugging devices inside residences or private vehicles, a warrant signed by the Home Secretary is required.

However, directed surveillance, where police or the intelligence services want to follow an individual in public or conceal a surveillance device without having to interfere with property - and that includes bugging a prison cell or a table in the visitor’s room - can be authorised “internally”. In the case of the police, authorisation can be granted by a superintendent.

Prison cells are not bugged routinely, but in the past there have been examples where conversations between terrorist suspects and other prisoners have been overhead by means of hidden listening devices.

In the recent trial of four Pakistani-born terrorists who pleaded guilty to plotting to kidnap and behead a British Muslim soldier, it was revealed that MI5 had entered the house of Parviz Khan, the ringleader, and planted bugs. That break-in would have required a warrant from the Home Secretary.

Although electronic bugs remain the staple diet of the surveillance community, the mobile phone has provided a revolution in eavesdropping techniques. They can be used as bugging devices themselves, simply by placing them covertly in an office or home being targeted and switching them on. Mobiles that double as listening devices can be bought on the internet.

Terrorists are fully aware that encrypted mobile phones are vulnerable to eavesdropping by the authorities. “If you can acquire the service number of the mobile phone, what’s called the IMEI number, located just under the battery, you can tap in to people's conversations,” Mr Marks said.

Terrorists try to foil the counter-terrorist surveillance experts by buying pay-as-you-go mobile phones which they use once and then throw away.

When it was alleged in 2004 that Kofi Annan, then United Nations Secretary-General, had had his offices bugged by American or British intelligence services, there was much speculation over whether the form of eavesdropping had been a planted mobile phone or an electronic bug.

However, there are other more sophisticated methods, including using laser technology. A laser beam bouncing off an office window can pick up the vibrations of conversation which can then be translated into speech.

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