Thousands of terrorism suspects are on the loose in Germany and there is no way to track them down, says the former head of counterterrorism for MI6, Richard Barrett.

His intervention comes as it was revealed the Berlin truck attack suspect Anis Amri was known to security services and police as a criminal with proven extremist leanings.

Barrett said Amri was just one of hundreds of “really extreme potential terrorists on the books” in Germany.

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“In addition to that though, if you include all the Lander (local regions) in Germany, they have about 7,000 live cases,” he told the Daily Mail.

“As you can imagine, that is an almost impossible number to control.”

Recent UK Home Office figures, however, threw into question what qualifies as a terrorism suspect in Britain after they showed that most UK terrorism arrests result in no charges or convictions.

The statistics show that the vast majority of suspects arrested in relation to terrorism over the last decade and a half are eventually released without charge.

Of 3,349 suspects from England and Wales arrested under terrorism legislation since the September 11 attacks, only around 18 percent were convicted of planning or facilitating violent acts.

A further 10 percent were arrested for terrorism but convicted of non-terrorism charges. The majority of those arrested in that period were British Muslims.

Liberal Democrat home affairs spokesman Lord Paddick, himself a former high-ranking police officer, told the Guardian Friday: “The police need to account for these figures. It is extremely worrying that there is such a high attrition rate between those arrested and those convicted of terrorism offenses.”