British police on Thursday charged a 26-year-old woman with inciting terror through Twitter and membership of the Islamic State group, and arrested a 15-year-old girl on suspicion of preparing an attack.

Police were also continuing an investigation into the disappearance of a family of 12, including three generations, believed to have travelled to Syria.

The 26-year-old woman was detained at Heathrow Airport after returning from Turkey.

The charges say that between September 30 and October 20 last year she published tweets intending people to be “directly encouraged or otherwise induced” to commit, prepare or instigate acts of terrorism.

Experts have said that Western women joining Islamic State militants are driven by the same ideological passion as many male recruits and should be seen potentially as dangerous cheerleaders for the cause.

A study by the London-based Institute for Strategic Dialogue earlier this year said that an estimated 550 Western women have travelled to Iraq and Syria.