The fight against espionage (spying) is one of the Security Service's oldest responsibilities. This threat was our main area of effort before we began to focus on terrorism, and it remains a major concern despite the end of the Cold War.
The threat of espionage did not end with the collapse of Soviet communism in the early 1990s. A number of countries are continuing to seek sensitive information from UK sources. In doing so, they are aiming to advance their own military, technological, political and economic programmes.
We estimate that at least 20 foreign intelligence services are currently operating in the UK against UK interests. The Russian and Chinese intelligence services are particularly active, and currently present the greatest concern. For instance, the number of Russian intelligence officers in London is at the same level as in Soviet times.
The threat against UK interests is not confined to the UK itself. A foreign intelligence service operates best in its own country. Some foreign services may therefore find it easier to target UK interests in their home countries, where they have much greater freedom of action than they would in the UK itself.
State intelligence services are not the only parties responsible for spying. Terrorist networks have sought to identify targets and vulnerabilities through the use of espionage methods, such as seeking to gain access to sensitive sites or material, and recruiting or placing sympathisers within organisations.
This section provides more information on the threat of espionage, the nature of the threat and our work to counter it. In the "notable spy cases" section, you will be able to read a number of original documents and images that illustrate the diversity and complexity of our counter-espionage work. Please click on one of the following links for more information.