The federal government is looking to reduce the share of funding it provides to the public school sector, according to angry state and territory education ministers who faced off with Christopher Pyne at a “very heated” meeting on Friday.

Far from allaying concerns over the federal government’s decision to rewrite the David Gonski-inspired funding system next year, Friday’s face-to-face discussion has further stoked anger from both Liberal and Labor colleagues who are demanding assurances their states will not be disadvantaged for having signed deals with the former government.

In a show of force, the education ministers from jurisdictions that signed a deal – the conservative-led New South Wales and Victoria and Labor-run South Australia, Tasmania and the Australian Capital Territory – jointly fronted the media in Sydney to demand the government meet its election pledge to honour signed agreements on school funding.

The NSW education minister, Adrian Piccoli, led the attack on his federal Coalition colleagues, saying pointedly that the Abbott government had broken its election promises and the “parents of the millions of children” had a right to be disappointed.

“The government made a promise, made a commitment, that there would be no broken promises under the government that they lead, and unfortunately that has not come to pass,” Piccoli said.

Pyne said this week he would match the $1.6bn total extra federal funding budgeted by Labor over the next four years, plus an extra $230m for Western Australia, Queensland and the Northern Territory – states which failed to reach a deal with the former government before the election. He has not repeated pledges that no school would be worse off.

States that signed up fear they will lose out in a redistribution of funding after 2014 as part of Pyne’s yet-to-be-developed new funding model.

Piccoli said there was uncertainty over who would bear the loss of any reduction in funding in the three years after 2014, revealing Pyne made comments “that would presume that that loss will have to be borne by public schools”.

“When asked for clarification about that, or to confirm that that was not the case, no such clarification was forthcoming, so not only do we have the uncertainty about the funding over six years, over four years – what that funding might be, even within four years – but the reduction in the funding, where’s that burden going to fall?,” said Piccoli, a National party minister.

“It’s been made very clear to me by the independent and Catholic sectors in NSW that they agree to the split of additional funds that we signed up to.”

Piccoli added: “I think that’s quite an incredible outcome, if reduced funding for states only comes out of public schools, not out of non-government schools.”

The Tasmanian education minister, Nick McKim, said Pyne “implied strongly” that he retained the capacity to renegotiate agreements, including on the funding share between government and non-government school sectors.

“This is a bombshell revelation that will rock the public education system in Australia to its core,” said McKim, who is also leader of the Greens in Tasmania.

“You wouldn’t have thought it possible; there is now less certainty at lunchtime today than there was when we walked into this meeting this morning. That is down to Christopher Pyne’s complete incapacity to offer any guarantees that satisfied these states.”

Pyne, who was standing nearby during the state ministers’ media conference, sought to downplay the conflict, saying there was nothing new in disputes between federal and state governments over funding and he expected ministers to argue strongly on behalf of their states.

Asked whether he was committed to a sector-blind approach to funding, Pyne said: “The sector-blind approach to funding was for disadvantaged students … I’m completely committed to a sector-blind approach on the loadings that might follow students whether they are in the public sector or the private sector, and that’s always what we’ve said.”

The reforms pursued by the former Labor federal government would see a base level of funding for each student, to be topped up by sector-blind loadings targeting specific categories of disadvantage. The benchmark funding for each student was to be adjusted in the non-government school sector, based on the capacity of parents to pay.

Catholic and independent schools have previously received the majority of government funding from the Commonwealth, while public schools have been largely funded by the states. The Gonski panel’s report, published last year, recommended a shift towards “more balanced funding roles”, with the Commonwealth assuming a greater role in funding government schools and the states taking on a greater role in funding non-government schools.

Australian Education Union national president Angelo Gavrielatos said the loadings represented only a small fraction of all the additional funding and he feared cuts to the base funding.

He said any suggestion that any such cut would be borne only by the public school sector was “an affront to every public school student, their teachers and their parents”, given the majority of disadvantaged students attended government schools.

“In all of my involvement over many years I’ve never seen a press conference where ministers – National party ministers, Liberal party ministers, a Greens minister and Labor ministers – have expressed such unity and such force with respect to the critical issue of funding,” Gavrielatos said.

Further comment is being sought from Pyne about his plans regarding federal funding shares for government and non-government schools. But he earlier told reporters in Sydney that the Coalition had made it clear it would match funding over four years, rather than the full six-year agreements signed by Labor.

“Every commitment that I have made, we are keeping, but we do need to sort out the [Bill] Shorten shambles I have been left,” Pyne said. “The idea there is a national funding model is quite frankly farcical and everyone knows it.”

Pyne said the former government had made different deals with different states when it came to annual indexation and the pace at which the new base level of funding would be reached.

His new model, to be revealed early next year, would be truly national and fair and would not treat students in some states as second class, he said.

Piccoli, chair of the ministerial council that met in Sydney, said states that signed a deal with the Commonwealth before the election did not oppose extra money flowing to the hold-out states, but this funding should not be taken away from the early adopters.

He said the recent funding agreements had brought peace to the long-running battles over funding of public schools and non-government schools, but the federal government had now “plunged education across this country into unnecessary uncertainty”.

“The bottom line is the current federal government made unequivocal promises that they would honour dollar for dollar agreements and funding signed by the states, and those signatory states are here as part of this press conference and we are very disappointed that they have so far made announcements that they are not going to honour those agreements,” Piccoli said.

The Victorian Liberal education minister, Martin Dixon, said the state was currently framing its budget for the 2014-15 financial year, but had no certainty about the level of federal support for schools in the 2015 calendar year.

An open letter to Pyne, signed by Gonski review panel members Ken Boston, Kathryn Greiner and Carmen Lawrence, as well as education advocates and charities, urges the federal government to stick by its election claim to be on a school funding “unity ticket” with Labor by keeping the new system for at least four years.

The prime minister, Tony Abbott, told reporters in Adelaide on Friday he could guarantee "that the money that was agreed to for next year will be fully delivered".

But the federal opposition education spokeswoman, Kate Ellis, said the prospect of public school funding cuts would revive a divisive debate pitting parent against parent, school against school and state against state.

"We know that some of the most disadvantaged schools in this country are run in the government system and they cannot be thrown aside by this cruel government who have already broken promise after promise when it comes to school funding," Ellis said in Adelaide.

The Greens' spokeswoman on schools, Penny Wright, said the Coalition's "elitist and shameful" plan went against the Gonski aim to end huge education gaps between the most and least privileged children.