Some of Britain's leading heroin smuggling suspects were protected from police investigations because they were working as informers for Customs, the BBC has been told.
Police suspected the informers were major heroin traffickers
In one case in 2001, Foreign Office diplomats moved to secure the release of an informer held in Germany on a warrant from the Greek authorities.
A former drugs specialist from the National Criminal Intelligence Service told Radio 4's File on 4 the treatment given to Andreas Antoniades made him "livid".
At the time the service - now amalgamated into the Serious Organised Crime Agency - was preparing to target claims he was bringing heroin into Britain.
It is common practice to use criminals as a source of information.
But the allegations suggest a special form of protection was being given to several people suspected by police of being leading importers of heroin.
"Customs told me he had been an informant and that he had been the best informant Customs ever had and what he had given the UK far exceeded the damage he had done, which was absolute rubbish," the former NCIS officer said.
"If you protect those people to get information about the guys who are dealing with a kilo, it really doesn't make the system work."
Mr Antoniades, 75, who is now believed to be living in Dubai, has never been convicted of any drugs offence.
But a string of people, some from the Turkish and Greek communities, have launched appeals against drug trafficking convictions based on his information.
A lawyer handling some of the cases said many informers regarded themselves, rightly or wrongly, as untouchables.
Siobhan Egan said the "balance of power between the informants and their handlers was corrupted".
"It is the only word you can use when you have a handful of informants who are working for a variety of agencies and these individuals are invariably criminally active," she said.
Customs declined to be interviewed by the BBC about the cases or the way informers are handled.
In a statement it said many of the cases looked at are historic and there are "complex issues involved and they are the subject of current consideration before the Court of Appeal".
It added: "Since these cases occurred there have been many changes in our handling procedures".
Greek Cypriot Mr Antoniades came to the UK in the late 1950s after working as an agent for British intelligence.
He was once jailed for four years after a shooting incident in west London.
He worked as an informer until the 1990s when reports emerged he was involved in drug trafficking.
In 2001, the Greek authorities accused him of trafficking and he was arrested in Germany on an extradition warrant.
Telegrams, formally sent by Foreign Secretary Jack Straw addressing Mr Antoniades' case, instructed diplomats to persuade the Germans to release him.
Diplomats were told to "press the case for Mr Antoniades' release immediately" with German state and federal justice ministers.
They were requested to point out that "a public trial in Greece would reveal Mr Antoniades' long career as an informant for Customs and Excise and put his life at risk from criminal elements".
A spokesman for Mr Straw, now Leader of the House of Commons, referred inquiries to the Foreign Office who in turn asked the BBC to contact Customs.
Hear the full story on Radio 4: File on 4 Tue 6 March at 2000 GMT