The Government has until the Autumn Budget to increase its funding for schools or risk strike action, an education leader has warned.
Kevin Courtney, general secretary of the National Union of Teachers (NUT), said its members were currently unlikely to resort to industrial action over what they perceive to be a chronic lack of funding for schools across the country.
But he said the profession "can't be expected just to sit back" while staff shortages and workload go unaddressed.
Speaking from the NUT annual conference in Cardiff, Mr Courtney said: "There would be nothing in any sense immoral in striking against these cuts.
"But that's not where our current planning horizon is.
"We will want the Government before Autumn Budget to announce they are putting significant resources in.
"But we can't be expected just to sit back and see class sizes going up; arts, dance, drama teachers being dismissed; children losing opportunities; and not think at some stage there would be an industrial response to that.
"If the Government doesn't shift, we think that's (striking) a likely scenario. I think we can build parental support, we can have a huge mobilisation."
Mr Courtney was speaking as it was announced that seven unions would lobby Parliament on June 6 protesting at school funding cuts.
NUT members will be joined by those from the Association of Teachers and Lecturers, the National Association of Head Teachers, the Association of School and College Leaders, Unison, the GMB and Unite, as well as parent group Rescue Our Schools.
Members will be lobbying MPs to introduce a fair funding formula which they say will help every child succeed; for Government to provide enough money to ensure that the formula supports schools properly; and reversing the 75% cut to the local authority education services grant which supports school improvement and pupils with the greatest needs.
Mr Courtney said: "I think this is a significant moment that the headteacher organisations are joining with us to call a lobby of Parliament.
"Up until this point, we've had different campaigns running. Now we are singing from the same hymn sheet."