Theresa May, who makes a major speech on Tuesday, has reached a moment of truth on Brexit. No one can blame her for the result of the referendum – she was a remainer – but from now on she takes responsibility for the consequences. If Britain not only leaves the EU but also loses free access to its main export market in Europe, our economy will become smaller and poorer.

Of course, she will argue that British exporters will not be barred from Europe’s vast marketplace, and technically that is true. But from now on we will be selling our goods and services on Europe’s terms; not as insiders but as competitors and rivals. Any deal that helps us will depend on Europe’s willingness to grant us preferential access.

Without favoured treatment, many exports to the EU could face tariffs in some cases of 10% or a lot more. Robust customs barriers will add significant export costs and expensive delays, and many exports of services will be blocked once we abandon Europe’s single regulatory rulebook. This means not just a hard Brexit but a destructive and harmful rupture that will, over time, reduce trade, shrink manufacturing investment and destroy jobs.

May will claim that this can be mitigated by special new trade arrangements between Britain and the EU. There are indeed options: an agreement that eliminates tariffs; a customs agreement to limit trade delays; mutual recognition of standards and regulations to smooth trade in services; and possibly an investment protection agreement. A transitional period could tide Britain over the lengthy time all this will take to negotiate and implement. But none of these will fall into place just because we want them to.

I used to negotiate such deals with foreign countries – “third countries” in EU parlance – with the same status as Britain will have. I can testify that so-called free trade agreements never come for free, they never cover all trade, and they are a devil’s own job to agree. As a rule, they are launched with ambition then conclude disappointingly many years later with both sides protecting their key interests.

President-elect Donald Trump’s offer of a “fair” and “rapid” trade deal with the US is welcome, but what would it look like in reality? Outside the EU, Britain would have little power to resist the imposition of US demands. The trade potential would largely be found in eliminating UK-US regulatory differences in goods and services, but Britain would be aligning itself with US technical standards rather than the other way round. So we would have to import hormone-treated beef and accept other US agricultural norms currently outlawed in the EU to protect consumers. And the more we aligned with the US, a matching regulatory gap would hit our European trade.

No doubt the government will continue to talk up the chances of this and other good trade deals, including with the EU, but there is only one way that Britain can enjoy the same tariff-free rights and regulatory privileges we currently have inside Europe’s single market – that is by joining the European Economic Area (EEA), as Norway did when it rejected EU membership. Hardline Brexiters oppose this, claiming that in voting to leave the EU, Britain also decided to leave the single market. But this isn’t true. The public were assured by leave campaigners that we would continue to have all the advantages of trade in the single market without EU membership. This outcome is only available to us in the EEA.

Yes, it would mean accepting rulings of the European court of justice in future disputes between the UK and the EU. It would also mean accepting mobility of labour between Britain and the EU. But how this principle is applied and how it operates in practice is not set in stone. There are growing calls in Europe for movement of labour to be reformed.

Already, labour market rules differ across the EU. Welfare entitlements vary. In many EU states, if you come you are expected to have a job. And the ability to work is not regarded as the same as the right to settle, with all the benefits and access to public services that come with it.

May should aim to negotiate arrangements with the EU, combined with changes in UK labour market legislation, which give us both continued free trade in Europe and greater control over labour mobility and employment rules.

Instead, the prime minister’s reading of the referendum is that the only thing the public cares about is immigration, and that immigration should now run economic policy even if it means the country becoming poorer as a result.

At the same time, she claims that the strength of our democracy and our identity as a nation is that we show respect for each other as fellow citizens. What about the half of the country that voted to leave the EU showing respect for the half that didn’t? If she really wanted to unite the country she would say “the country voted to leave by a small margin but parliament’s job is to find the least harmful way of doing this”.

Instead she has brushed aside advice from her civil servants, threatened the EU with tax and trade wars, bullied business leaders into keeping quiet about the consequences, and allows anyone with a different view to be treated as a heretic. The correct response from those in all parties who care deeply about the national interest is to cast partisan politics aside and make their voices heard in the country and in parliament. This is going to hang over British politics for the next decade at least. If the prime minister is not serious about unifying the country, others should be.