Hizb ut-Tahrir (HuT), which is legal in Britain but banned in Germany and much of the Middle East, advised Shabina Begum, a 15-year-old orphan. Her case, which was funded by legal aid, was thrown out by the High Court last week.

Mainstream Muslim leaders reacted angrily to news of extremist involvement in the case. They fear it risks stirring up the sort of controversy sparked in France when the government banned the wearing of the hijab, or headscarf, in school.

Khalid Mahmood, Labour MP for Birmingham’s Perry Bar constituency, said: “Most Muslims are happy with the existing dress code. I think they (HuT) are trying to pick a fight. The Home Office needs to look at some of their activities. At the moment they are very close to the edge.”

Mahmood said HuT’s role was particularly disturbing because of Begum’s vulnerability. She was 13 when, in September 2002, she was sent home from Denbigh high school in Luton for wearing a jilbab, an ankle-length dress that leaves only the face and hands visible.

Begum, who was regarded as a promising pupil, was orphaned last April with the death of her mother. Her father had died in 1992. Her 21-year-old brother, Shuweb Rahman, who helped her bring the case, is an HuT supporter.

Although she still intends to sit seven GCSEs next year, she has not attended any classes for almost two years. Her teachers have sent her schoolwork and have taught her outside school hours on a small number of occasions.

Dr Imran Waheed, an HuT spokesman, confirmed that leading activists had encouraged Begum in the dispute. “Our members in Luton have consistently advised Shabina and her family to stand up for her right to an education and her right to observe the Islamic ordinances, including the wearing of the jilbab,” he said in a statement.

He emphasised that the group had not contributed financially towards the legal action or to her family.

According to Dr Nazreen Nawaz, also an HuT spokesman, one of the group’s supporters, Rebekha Khan, 23, has been in contact with Begum for the past two years. This weekend, Khan played down her role: “The first time I met Shabina was at an Islamic event two years ago. It was clear to me even then that she was already very orientated to Islam.”

Mahmood, who has in the past likened HuT to the British National party, said it had a record of targeting young people in schools and universities to lure them away from the mainstream of the Muslim community in Britain. “It is important that social services look into that role,” he said.

Denbigh High school was an unlikely target for criticism. Almost 80% of its 1,000 pupils are Muslim, and its dress code, which allows pupils to wear a shalwar kameez (trousers and a long tunic), was introduced with the support of the town’s Council of Mosques.

The school argued that going further, by permitting the jilbab, might create divisions by implying that those who did not wear it were not as devout as those who did. It also suggested that the garment posed a safety hazard.

Earlier this year, another Luton school, Icknield High, was targeted by extremists after the head teacher, Keith Ford, insisted that Muslim girls should not wear hijabs. Ford took early retirement, although insisted he would not be forced to “retire over a matter of a piece of cloth”.

According to Geoff Lambert, then chairman of the board of governors, the picket by the radical Muslim group al-Muhajiroun was counterproductive. He said the governors had already decided on legal advice in January that they had to permit the wearing of the hijab.