There is no intrinsic reason why Brexit should be difficult or damaging, but the EU itself has so far demonstrated it wants to make it so; and it has co-opted the CBI, parts of the City and, it seems, the Treasury to assist. They are legitimising EU threats of economic disruption. We are fast reaching the point when the prime minister should assert the authority of her office over the negotiations and call time. A clean break in 2019 would be preferable to the mess they want to draw us into. To be prepared for no trade deal with the EU, Whitehall must focus on planning and readiness without further prevarication.

The Florence speech sets out a credible and conciliatory framework for the negotiations, but more concessions would be damaging, expensive and unnecessary, and would prolong economic uncertainty, damage confidence and enrage leave voters, threatening the stability of the government. Many hardcore leave voters, some 30% of electors, were drawn back to the mainstream parties at the last election, because both parties were clear the UK is leaving the EU. Those voters would feel betrayed, and the broad majority of Conservative MPs would feel that the establishment had imposed its will on the government, despite what the prime minister really wants. No government could sustain a policy of Brexit in name only, which is what some seem determined to pursue, flying in the face of the clear referendum result.

Only a fraction of the UK economy is dependent upon EU membership. The majority is either domestic or non-EU export

An interim period would allow time for processes and institutions to be adapted or established so that by the end of two years or so, a comprehensive trade agreement can be implemented. As the prime minister has made clear, it must be clear what is being implemented. The EU should stop obstructing these discussions about an agreement, which should be relatively easy to agree in principle, since the UK and the EU will have identical regulatory systems on the day of Brexit, so both sides can aim at maximum mutual recognition of different sectors.

A soft customs frontier should be established from the day the UK leaves the EU. There is no need for zealous enforcement by the UK or by the EU of the “rules of origin” that govern the international trade in goods. There is no need for tariffs if the EU and the UK have jointly notified the World Trade Organisation of our intention to implement a zero-tariff comprehensive trade deal, and of our special customs agreement for the interim. If everyone was intent on being reasonable, the practical or legal problems would be surmountable. However, the EU is determined to be anything but. It refused to enter any discussions at all until the UK had invoked article 50. We did so, and still the EU refused to discuss the future trade relationship, insisting first on “sufficient progress” on citizenship, money and the Northern Ireland border.

Following the Florence speech, where the prime minister conceded, among other things, that money would be on the table for the interim period, the EU is still blocking trade talks. Treasury officials are briefing that we must stay in the customs union for the interim period. They insist we must continue to pay billions of pounds to subsidise other EU states, while we must accept “full regulatory and judicial oversight”, which means we would have to submit to any new European court rulings and new directives and regulations without any say over those new laws. Worst of all, matters are already being decided between the UK and the EU, such as on the dividing of tariff-rate quotas for imports from non-EU countries (for instance, on New Zealand lamb) the joint presentation of which could hobble the UK’s ability to make meaningful trade deals with non-EU countries into the future. That is why the US and other nations have challenged the UK and the EU on this, saying they “cannot accept” this unilateral splitting of tariff-rate quotas without the EU and the UK negotiating separately with all the countries concerned.

The Treasury seems unable to hear any voices except those that reinforce their preconceptions. It seems blind to the facts, preoccupied with preserving “access” to the EU market seemingly at any cost. People such as James Dyson or Anthony Bamford of JCB are ignored. Only a fraction of the UK economy is dependent on membership of the EU. The majority of the economy is either domestic or non-EU export. It will be a benefit to the public finances and to our trade balance when the exchequer is relieved of contributing to the EU budget (a net £10bn a year and growing, and a total of £390bn since we joined, at current prices). A small fraction of the billions demanded by the EU would be far better spent on being ready to leave without a trade deal if no sensible agreement can be reached.

• Bernard Jenkin MP is chair of the steering committee of the European Research Group of MPs