Nine-year-old Lloyd Bent will spend tomorrow travelling from his home in London to visit his father, also named Lloyd, who is being held in Winson Green high security prison in Birmingham.

Lloyd, like his mother Marcia and step-brothers Jermaine, 10, and Leo, 8, has been spending most Saturdays like this since his father was detained in prison in February under immigration rules. A grave, dignified little boy, Lloyd finds it impossible to understand why his father is in prison when he has not committed a crime.

The three boys and their mother spend much of their remaining free time campaigning for their father’s release - handing out leaflets, collecting signatures for petitions, accosting any passer by who will listen - and the family’s plight has attracted the attention of local teachers through the National Union of Teachers.

The headteacher of the boys’ primary did not want the school identified, but she said that a letter of support from herself and the governing body had been dispatched to the Home Office.

Lloyd Bent senior has never been convicted of any crime, either in the UK or in his native Jamaica. But, through what appears to be a combination of bad advice and failure to appreciate the complexities of British immigration law, Mr Bent is technically a criminal and can be confined indefinitely in prison - the same one that housed serial killer Fred West.

Mr Bent arrived here in 1994 on a working-holiday visa, and in 1996 married Marcia, a British citizen. In February 96 he returned to Jamaica for three weeks for the funeral of his brother, having taken the precaution of asking a law centre whether there would be any difficulty coming back into the country. He claims that he was told that there would be no problem.

Re-entry to Britain as a visitor was refused on the grounds that he was married to a British citizen and Lloyd Bent then paid Pounds 350 to an “immigration specialist” who advised him, disastrously as it turned out, to apply for political asylum. This was refused, and, says his solicitor Martin Hoare, undermined the credibility of his case.

Mr Bent was detained at Rochester detention centre in February, where, he claims, he was severely beaten up by staff before being transferred to Winson Green in March. The alleged assault is the subject of a separate legal case.

Martin Powell Davies of Lewisham NUT says the case highlights the intense stress to which children in these circumstances are subjected. “Teachers are increasingly becoming involved in supporting these children and their families because they are often the first to notice the effects on children’s behaviour and performance. One of the problems for these families is that they are often have very few contacts in this country. The children’s teacher is often the only person they can ask for help.”

Teachers at Haggerston school in north London, for example, have taken up the cases of two families of children at the school: Mansanga and Feliciana Nanga, and Muyeke Lemba, all of whom have parents under threat of deportation back to Angola.

Marcia Bent says she has not noticed any softening in the Home Office’s attitude since Labour came to power, but last week she was finally granted an interview with immigration minister Michael O’Brien and was able to put her case. He was not however able to give her any idea when a decision would be reached.

The effect of all this on Lloyd Bent’s son and stepsons is obvious. Asked how he feels about his father’s detention, nine-year-old Lloyd, unprompted, immediately launches into a description of the night his father was taken away. Now, he says: “I can’t sleep, I’m always scared someone is going to break in while my Dad’s not here and take us all away somewhere.”

Marcia Bent says that Jermaine has become withdrawn and cynical at school and suspicious of the white teachers. His ambition is to become an MP. Leo, the youngest, is cheerful and inquisitive, but his conversation constantly reverts to the absence of his stepfather.

One of the walls in the family’s south London flat is covered with letters from prison; decorated with drawings of hearts and flowers, and full of anxious enquiries about the family’s welfare and the boys’ school work. They make painful reading. Lloyd Bent and his family are paying a high price for his failure to appreciate the subtleties of British immigration bureaucracy.