Shamima Begum’s attempt to overturn the decision to revoke her UK citizenship is being deliberately delayed, according to her family’s lawyer, to give police time to charge the former Islamic State member with a terrorism offence.

Almost six months have passed since Begum lodged an appeal against the decision, by the former home secretary, Sajid Javid. Yet the 19-year-old, who joined Isis aged 15 and remains in Syria, is still waiting for a date to be set for the appeal.

The lawyer for Begum’s family believes the “unusual” delay is to give the Metropolitan Police time to build a criminal case that can be used to impose strict conditions on her if she were to be brought to the UK for the hearing. Without conditions, Begum would likely be able to move freely around the UK, something likely to prompt a fierce backlash from elements of the media.

Last week, Scotland Yard applied for an application under the Terrorism Act demanding access to notes from three media organisations about Begum. One target appears to be Anthony Loyd, the Times journalist who found Begum in a refugee camp in February and reported her lack of regret over having joined Isis.

Tasnime Akunjee, Begum’s family’s lawyer, said he believed the police were investigating the teenager before repatriating her for the hearing. “The police are trying to build a criminal case against her.”

Meanwhile, it has emerged that no progress has been made to bring the children of British fighters in Syria back to the UK, despite a promise by the government five months ago.

In March, then foreign secretary Jeremy Hunt said the Foreign Office was working with the Department for International Development on contacting and removing British children in the Syrian camps.

However, no British minors have since been removed, with a Foreign Office spokesperson saying that “there are things we cannot do because we do not have a consular presence within Syria”.

A Commons briefing paper on returning fighters from Syria recently said that acquiring material against Britons in Syria was fraught with “difficulties in obtaining evidence of conduct that has taken place abroad in territory without a functioning criminal justice system that UK authorities could cooperate with”.

Currently more than 70,000 people are living in al-Hawl camp in northern Syria, where Begum was found, with most from Syria and Iraq and more than 90% women and children.

It is not known how many Britons are on official “humanitarian lists” in al-Hawl but it is believed at least 20 British women and children are being held in camps run by Kurdish forces after fleeing Isis territories.

“We are particularly concerned about children who are living in the camps without their parents or habitual guardians, as well as other especially vulnerable persons. Identifying and following up on these cases is in progress,” said an International Committee of the Red Cross spokesperson.