The education minister has warned it would immediately cost his department £20m a year at a time when it is already struggling to balance its budget.
Departmental officials due to be questioned about integrated education on Wednesday will now not appear.
The prime minister Andy Burnham said that he did not have “unlimited funds”.
All five of Northern Ireland's teaching unions will take part in the action from Monday, 28 September.
Guidelines stipulate that school uniform policies for the 2027/28 school year onwards must include the option for girls to wear trousers.
Northern Ireland Executive parties had rejected an initial £1.1bn package over three years that was offered on Monday.
The Education Authority said "arrangements to implement the change will begin immediately".
Northern Ireland's five teaching unions voted for industrial action if their grievances concerning workload were not addressed.
Paul Givan has said there is a need for "real and frank conversations" about the sector and how "real integration" is achieved.
