The next few weeks are likely to be as important for Scotland’s future as any in our recent history. In the negotiations between the UK government and the European Union, the future of our economy and our rights and the prospects for future generations will be at stake.

So far, as exemplified by the speech given last week by Boris Johnson, it is the Tory hardliners who have been calling the shots. Despite the overwhelming vote in Scotland for remain, the Tories are currently pursuing a hard Brexit no matter the cost to jobs and living standards. But Brexit doesn’t just pose a threat to our economy, it is now a threat to the hard-won devolution settlement that people in Scotland voted for so decisively in 1997.

Just as talks between the UK government and the EU are reaching a crucial stage, so are negotiations between the devolved governments and the UK government. On Thursday ministers from the various administrations will meet again in London. As with so many other aspects of Brexit, this debate is too often shrouded in jargon, which can hinder understanding of what is really going on.

One of the terms often used is whether there should be “UK frameworks”. The point here is whether or not arrangements in certain devolved policy areas should be put in place on a UK-wide basis in the event of Brexit.

The Scottish government has always been clear that we are not opposed in principle to such arrangements, where they are in Scotland’s interests. But because we are talking here about devolved powers, it must be for the Scottish parliament to decide if and when these powers should be exercised on a co-operative basis across the UK in the future. However, the UK government’s EU withdrawal bill drives a coach and horses through this principle and the devolution settlement itself.

This bill should be a relatively uncontroversial technical piece of work to prepare Scottish and UK laws in the event the UK leaves the EU. Instead it gives Westminster control of what happens in all of the devolved policy areas, such as farming, fishing and the environment, related to our EU membership and will allow the UK government to impose whatever arrangements it sees fit, regardless of the views of the Scottish parliament.

This is why the Scottish government, the Welsh government, the Scottish parliament and a host of academics and legal experts say the bill is incompatible with devolution. It is important to emphasise that at stake is what happens to policy areas that are already devolved and currently subject to EU law. The dispute over the withdrawal bill is not about further devolution or more powers for the parliament (that is an important related debate) but about existing devolved areas.

Far from the “powers bonanza” promised by the UK government, this is entirely about Westminster’s plan to constrain devolved powers, not add to them. Last summer we were startled to discover the UK government had its eyes on 111 devolved policy areas. Under these proposals any area that, in the UK government’s language, “intersected” with EU law would be controlled by Westminster unless and until the UK government decided that such control was no longer necessary in pursuit of its policy agenda.

Extraordinarily this approach even extended to aspects of the justice system in Scotland, a system that has always operated independently from England. This approach and the list of functions was of course completely absurd. But it betrays a very clear attitude, that the Westminster government, not the Scottish parliament, should be in control of every aspect of the Brexit process, including determining the future for Scotland in wholly devolved areas.

After several months of meetings, the UK government has cut the list down. But make no mistake, although the list is shorter the demand remains the same: Westminster must decide. This remains a power grab in critical areas at the heart of the devolution settlement, areas such as fishing, farming, GM crops, vital aspects of environmental quality, food standards and state aid.

This would mean, for example, that if the Tories wanted to lower food standards we could do little to prevent it. If they wanted to reduce environmental quality requirements we would be similarly powerless.

If Brexit happens there will be years of negotiations with the EU and other nations on trade deals that could see guaranteed access to UK markets for chlorinated chicken and hormone-injected beef and oblige us to open up our health service to private suppliers.

The UK government says we don’t have to worry about any of this because it hopes to consult the Scottish government. But if “consulting” had any meaning in relation to Brexit we would not be facing being dragged out of the single market against our will. We know from bitter experience that we must continually reassert the rights and powers of the Scottish parliament. Just as we faced plans for a £7bn cash grab only two years ago, we will instead see a continued power grab now.

We are simply seeking to protect the devolution settlement and to ensure the Scottish parliament has the powers in full that the people of Scotland voted for.

That would enable this and future Scottish governments and parliaments to act differently in Scotland when that is the right thing to do, and also to secure the best deal for Scotland in UK-wide negotiations and frameworks when that is in our national interest.

Despite our opposition to Brexit we have shown a willingness to compromise by proposing amendments to the withdrawal bill, but these were rejected by the Tories at Westminster. Despite promising to bring forward its own changes in the House of Commons, the UK government failed to do so. If at this late stage it changes its mind, and accepts that co-operation must be agreed, not imposed by Westminster, then a way forward can be found. But Westminster should be in no doubt that this is an absolute condition. We will never recommend giving consent to a bill that undermines the principles of devolution and the powers of the Scottish parliament that the people of Scotland campaigned for so long to win.

• John Swinney, a Scottish National party MSP for Perthshire North, is Scotland’s deputy first minister and cabinet secretary for education and skills; Michael Russell is a Scottish National party MSP for Argyll and Bute and minister for UK negotiations on Scotland’s place in Europe