An alleged plot to oust some Birmingham head teachers and make their schools adhere to more Islamic principles is being investigated, it has emerged.
A letter detailing the plan, known as "Operation Trojan Horse", claims responsibility for leadership changes at four schools.
These schools are Adderley Primary, Saltley School, Park View School and Regents Park Community Primary School.
Saltley's head teacher resigned last year after a critical Ofsted report.
Inspectors said there was a "dysfunctional" relationship between head teacher Balwant Bains and governors which was hindering the school.
The letter, which purports to outline "Operation Trojan Horse", has subsequently been sent to at least another 12 schools in the city - all believed to be vulnerable to takeover.
It states that parents could be encouraged to turn against the leadership team if they are told the school is "corrupting their children with sex education, teaching about homosexuals, making their children pray Christian prayers and [carrying out] mixed swimming and sport".
Among various claims in the letter is one that the group has "caused a great amount of organised disruption in Birmingham and as a result we have our own academies and are on the way to getting rid of more head teachers and taking over their schools".
The head teachers of the schools met Birmingham City Council on Thursday to discuss their concerns.
The letter, seen by the BBC, was apparently written by someone in Birmingham to a contact in Bradford, and goes on to outline ways and means by which schools can be taken over.
It says: "We have an obligation to our children to fulfil our roles and ensure these schools are run on Islamic principles."
It is understood that would mean a greater emphasis on religious studies, as well as girls and boys being taught separately in some classes.
The letter implies these methods have already been put into action and urges the recipient to use Ofsted reports to identify schools in predominantly Muslim areas which are struggling.
It adds: ''Operation 'Trojan Horse' has been very carefully thought through and is tried and tested within Birmingham, implementing it in Bradford will not be difficult for you."
It says that Salafi parents should be enlisted to help, because they are regarded as a more orthodox branch of Islam and would be more likely to be willing to help.
It was sent to the city council in 2013 and has led to a number of investigations. Part of the inquiry will focus on whether the plot is genuine or fake.
A Birmingham City Council spokesperson confirmed the letters had been received and that an investigation was ongoing.
'Not police matter'
The Department for Education's (DfE) Extremist Unit is also involved and the West Midlands Police Counter-Terrorism Unit has also looked into the case after being handed the letter in December 2013.
Supt Sue Southern, head of the unit, said it was decided the allegations in the letter were "not a matter for the police".
The National Association of Head Teachers (NAHT) said it had "received some anonymous letters in February which claimed that an extremist religious group was trying to engineer the sacking of head teachers who did not promote the group's ideals".
It said it was working with the police, the Department for Education and Birmingham City Council to investigate the claim.
Russell Hobby, NAHT general secretary, said the union took the allegations "extremely seriously".
Liam Byrne, Labour MP for Hodge Hill, said he had held urgent talks with Ofsted, City Council officials, the office of Michael Gove and DfE officials.
"These are deeply disturbing allegations, which is why Ofsted has been called in," he said.
"I have demanded that the second we have results from those inspections, both the city council and the Secretary of State take immediate action."