An investigation into an alleged "Trojan Horse" plot by some Muslims to seize control of schools is to get a staffing boost after a significant number of people came forward to raise fresh concerns.

Birmingham City Council said it had received a large amount of correspondence from teachers, governors and parents since launching an inquiry into a letter claiming responsibility for "disrupting" the leadership of several schools.

The Department for Education (DfE) is running a separate investigation, parallel to the local authority probe, covering academies which do not fall under council control.

In an official letter to all the city's headteachers, Birmingham City Council said: "Over recent weeks, as a result of these matters, we have had a significant number of teachers, governors and parents write to us with past and present concerns about governance and leadership in schools and we are taking all of these very seriously.

"We will shortly announce details of some additional staffing to review all this material."

The alleged plot first came to light after an anonymous letter purporting to be from one of the scheme's architects, and apparently written to an unidentified recipient in Bradford, was sent to the council and the press.

Within the unsigned and undated letter is a five-point plan detailing the process behind what is referred to in the document as Operation Trojan Horse, setting out a long-term strategy to ensure "schools are run on Islamic principles", and replacing unco-operative headteachers.

The unidentified author claims the plot has already caused "a great amount of organised disruption in Birmingham" and explains how it has been used in attempts to force leadership changes at three city schools.

However, the document also contains factual inaccuracies about some of the changes it alleges have happened as a result.

The Park View Education Trust, which is referred to in the letter and runs three of the schools now being investigated, has denied all the allegations.

The leader of Birmingham council Sir Albert Bore has previously labelled the letter's contents "defamatory" but made it clear the local authority will "act accordingly" if evidence of any wrong-doing is brought to light from the investigation.

The council's letter to headteachers, signed by Sir Albert and senior children's and education services officials, said: "We owe parents and children clarity and confidence about the highest standards of governance and leadership that we expect, and that they deserve, in our city's schools."

It reads: "The lack of a council comment on the defamatory matters contained in the Trojan Horse letters should not be misinterpreted as a disregard for these issues."

A DfE spokesman said: "We are aware of serious allegations made in relation to a number of schools in Birmingham.

"Officials from the department have visited Park View, Nansen Primary and Golden Hillock schools, all part of the Park View Education Trust.

"Separately, Birmingham City Council is investigating allegations made in relation to some local authority schools. Both investigations are ongoing.

"All schools are subject to a tough inspection framework and must meet the high standards and requirements rightly expected.

"We will not hesitate to take firm action if these are not being met and where we become aware of issues of concern we will move quickly to resolve these.

"We are maintaining close contact with a number of parties, including the police and Birmingham City Council.

"It would be inappropriate to comment further at this stage."