Guardian News and Media/London

The family of the Woolwich murder suspect Michael Adebolajo say he had predicted he would be attacked by prison officers in Belmarsh prison before he was allegedly assaulted last week.

His relatives have accused prison officers at the high-security establishment of assaulting the 28-year-old in what they claim was an unprovoked attack on July 17 in which he reportedly lost two teeth while being restrained.

Prison officers confirmed that a prisoner had been subjected to “approved techniques called ‘control and restraint’” that day but deny any wrongdoing.

In a statement, the Prison Officers Association (POA) said: “Our members strenuously deny any wrongdoing and the POA will be supporting them legally and emotionally during this difficult time. The use of restraint is only used where necessary when dealing with incidents up and down the country.”

The London Metropolitan police are investigating the incident at the prison, where Adebolajo is being held on remand with Michael Adebowale. They are both charged with the murder of Fusilier Lee Rigby in Woolwich.

According to CagePrisoners, a charity supporting prisoners’ relatives, family sources say Adebolajo had previously complained that prison officers had been waiting for opportunities to harm him.

Relatives claim a guard assaulted him without provocation, knocking out two teeth and smashing his head against a window.

Arnaud Mafille, casework officer at CagePrisoners, said: “Whilst we understand that an investigation into the conduct of the officer under question is now under way, one must ask how such allegations have emanated from a man placed within an isolated prison unit.”

The POA said it would fully co-operate with any police investigation, and expected officers to be fully exonerated. But a spokesman criticised the ministry of justice, saying it had done “little to avoid sensationalist reporting” of the incident over the last 48 hours.

The ministry of justice has refused to confirm reports that Adebolajo suffered injuries to his mouth or was targeted by prison officers. A spokeswoman said: “The police are investigating an incident that took place at HMP Belmarsh on July 17. It would be inappropriate to comment while the investigation (is) ongoing.”

Adebolajo, from Romford, is accused of hacking Rigby to death near Woolwich barracks in south-east London on May 22. Adebowale, 22, from Greenwich, has been accused alongside him. The trial is due to begin on November 18. Adebolajo faces further charges related to the attempted murder of two police officers and possession of a firearm with intent to cause others to believe violence would be used.