This is possibly the last Sunday afternoon that a devoted father will spend with his five children. Before dawn on Tuesday morning, officials from the Home Office will arrive at his house and escort him to Gatwick airport and a one-way flight to the country of his birth, Algeria.

He is not being forcibly deported; he has decided to leave voluntarily. “I feel like I’m torturing my own family by staying with them,” he told me. “The only way to protect them is to leave.”

Detainee “A” is one of the men certified as terrorists by David Blunkett after the terrorist attacks of September 11, 2001 and who became central characters in the moral and legal debates surrounding Britain’s domestic “war on terror”. He was one of the “Belmarsh 12”, jailed for more than three years without trial and whose incarceration was declared an illegal breach of the European convention on human rights.

Since then, Mr A, as he calls himself, has lived under various levels of house arrest — again without a trial — initially under “control order” legislation that was hurriedly introduced in response to the law lords’ ruling. The decision last month by a handful of detainees to return to Algeria has thrown new light on the secretive, unreported lives of “controllees” who have been left decaying in a legal purgatory for more than five years.

Softly spoken and thoughtful, Mr A’s fluent English is peppered with cockney slang. Britain has been his home for 17 years; his children were all born here. His eldest is preparing for his GCSEs this summer.

Under restrictions sought by the Home Office, he must remain indoors for 22 hours a day. His electronic tag tells a private monitoring company if he leaves the house during curfew hours and alerts the police. No one is allowed into his house without Home Office clearance and he is not allowed a mobile or access to the internet.

The restrictions are riddled with farce; I can visit the house and conduct an interview on his doorstep, the frame of the door forming an invisible barrier that neither of us can cross. He can meet whom he wants at his local mosque and travel on public transport unhindered. Yet the government maintains that secret intelligence has shown Detainee A to be a dangerous international terrorist who supports Al-Qaeda.

He was also accused of supplying satellite phones to extremists and of giving a sleeping bag and a pair of boots to Chechen rebels. He does not deny the accusation but says he was doing charitable work for the people of Chechnya: “It’s a Muslim’s duty to help other less fortunate brothers and sisters.”

It was A’s relations with his children that finally made him decide to leave. Their schoolwork had begun to suffer because they could not use the internet at home. His eldest children began to resent him: “You are useless for society, for your community, for your kids, you know. I can’t even do shopping for my wife.”

Gareth Peirce, Detainee A’s defence lawyer, says she is stunned by the increased use of intelligence to detain suspects, when neither the accused nor their lawyers are told what the information is: “We are debarred from knowing anything.”

There are now some 40 men held under partial house arrest, either under control orders or bail conditions imposed by the secretive Special Immigration Appeals Commission. Some of the orders seem to make little sense: Mahmoud Abu Rideh, a terrorist suspect, is subject to restrictions including a 12-hour curfew, but can still travel to Parliament Square.

In December 2001 he was detained under anti-terrorism laws, accused of supporting and raising funds for international terrorist groups. He was moved to Broadmoor hospital in July 2002, but was granted bail in January 2005. At the time, the Special Immigration Appeals Commission held that Mr Rideh “remains rightly certified as a suspected international terrorist who is, on sound grounds, believed to be a risk to national security”.

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