At least one or two teachers in every secondary school will have their salaries cut under a new pay system, headteachers were told this week.
They heard that plans to replace management allowances with teaching and learning responsibility (TLR) payments will mean "outright losers".
The reforms introduce two new payment ranges of pound;2,250-pound;5,500 and pound;6,500-pound;11,000.
The new payments have already been agreed by the Government, employers and most teacher unions, although the School Teachers' Review Body has yet to approve them.
Under a separate deal agreed in 2003, headline teachers' pay will rise by 2.5 per cent in April and 3.25 per cent in September The warning of pay cuts came from John Bedford, head of education personnel at Manchester city council, who said heads needed to start preparing staff.
"It is going to be almost impossible to drive this through without there being casualties," he told Leicestershire high school heads on Tuesday.
"The only refuge you will have is in the fairness and transparency of the approach you take. Time is your enemy and time is your friend in this. I think, if left until September, it will become very difficult."
The new system will require all schools to review staffing structures by the end of the year and implement them by the end of 2008.
But continuing delays in the publication of the review body's report, which was delivered to the Department for Education and Skills on January 21, means full details of the pay changes are not yet available.
Mr Bedford said that, with the introduction of the final phase of the workforce agreement taking place at the same time, heads would have an autumn term so hectic that they would remember it for years.
They were mistaken if they thought would be able to simply convert management allowance payments into TLR payments, he told the seminar organised by the Head Support pay and conditions consultancy.
The new payments had been set at levels that did not match the old allowances so as to drive heads into a complete rethink of their staffing structures to take account of workforce remodelling.
He said education authority officials had reacted with horror when told that schools would only have until December 31 to consult on and publish their new structures.
Nigel Middleton, Head Support director, said heads needed to begin to brief staff on restructuring by April or May.
But that would depend on further details about the allowances being published soon by the review body. The longer the STRB report was delayed the less likely that was to happen.
John Thomas, headteacher of Mount Grace high, Hinckley, Leicestershire, said: "We are not being given enough time. It is a matter of months to establish a structure that will underpin the school for the foreseeable future.
"The teacher at the chalkface will feel this is coming from nowhere."