Schools planning to become academies could see their applications thwarted if heads recognise unions and sign up to national pay and conditions for staff, the Government has confirmed.
The Department for Education has told The TES that a head's decision to sign up to a standard TUC recognition agreement could be "one of the factors that leads to the school not being granted academy status".
Ministers said they consider flexibility on staff pay to be one of the "key freedoms" of academy status, even though a large number of existing academies already keep to the national pay and conditions framework.
Heads who have applied for academy status received a letter from schools minister Lord Hill at the end of last year. It said that if a school signed up to a standard agreement being promoted by the NASUWT, it would be "a significant factor in the assessment the secretary of state will make before deciding whether or not to enter into a funding agreement for an academy".
The news comes as a poll of nearly 500 heads by the Association of School and College Leaders (ASCL) found 70 per cent thought the existing national pay and conditions framework contained adequate flexibility and should be retained in current form.
Unions are already anticipating an assault on the national pay and condition framework across all school types, after education secretary Michael Gove said he wanted to introduce more flexibility for heads to pay what they want.
Chris Keates, general secretary of the NASUWT, said in an angry response to Lord Hill: "This 'key freedom' must include the freedom for an academy to abide by the national framework if that is what the academy decides is best for its staff.
"There is nothing in the Academies Act that requires the national framework to be abandoned; it is wholly misleading and disingenuous to suggest otherwise."
Ms Keates is also furious that Lord Hill's letter told schools they have no obligation to consult with unions over academy conversions, or suspend conversions to allow consultation with unions.
He wrote: "The responsibility of the governing body of your schools is to ensure that there has been consultation with whomever they think is appropriate about whether to convert. There is therefore no reason for schools to agree to a request from NASUWT to halt or amend the conversion process."
Ms Keates replied: "Any suggestion that the NASUWT, a recognised trade union, where it has members at the school concerned, would not be such an 'appropriate' person is quite frankly ludicrous."
At heads' union the NAHT, general secretary Russell Hobby said members who had received the letter were fearful that signing up to national pay and conditions would jeopardise their chances of getting academy status.
He said: "I don't think it's a good idea for heads to have their hands tied on pay and conditions, as the opportunity to innovate around conditions like the school day is one of the most interesting bits about academy status. Equally, it's not really ideal for the Department to tell heads how to manage their workforce either."
Brian Lightman, general secretary of the ASCL, added: "The Government has consistently said one of the freedoms is they can make their own decisions and they should be able to do what they believe to be right.
"The current system has a very large number of flexibilities, and although schools don't have to sign up to this particular agreement we would advise them to talk to the unions and find a way of working with them."
However, the Government this week insisted the letter was only written to "clarify" to concerned headteachers that they were not obliged to sign any union recognition agreement if they did not want to.
A spokesman for the DfE said: "Schools are entirely free to choose to sign up to the NASUWTTUC agreement, but it could be one of the factors that leads to the school not being granted academy status."
He added that it is up to heads and governing bodies to decide on who they need to consult over academy conversion and whether or not that includes unions.