Twelve men arrested in a series of anti-terrorist raids involving hundreds of officers across northwest England earlier this month will not face charges due to insufficient evidence, British prosecutors said Wednesday.

Eleven of the arrested men, all of them Pakistani nationals, face deportation and have been transferred into the custody of British immigration authorities.

The men were arrested in a series of raids on April 8 in Manchester, Liverpool and the surrounding area, 320 kilometres northwest of London.

While British officials haven't provided any details about what the men were suspected of doing, Prime Minister Gordon Brown has previously said authorities were on to "a very big terrorist plot."

But British Muslim groups immediately called on the government to admit it had made a mistake with the arrests, and further complicated the case by making prejudicial and "premature" remarks to the media.

Bnayat Bunglawala, of the Muslim Council of Britain, told the BBC the government had been "dishonourable" over the way it had dealt with the men.

"Politics should not be interfering with what is primarily a legal process," he said.

The arrests have strained relations between Pakistan and Britain, and raised questions over Britain's student visa system. Ten out of the 12 arrested were in the country on student visas.

The operation also led to the resignation of Britain's top counterterrorism officer, Bob Quick, after he inadvertently revealed information on the case a day ahead of the raids by being photographed carrying confidential documents to a meeting with the prime minister.

With files from The Associated Press