For the last half century trade has never been a major part of domestic politics in Britain or indeed in Europe.

Unlike the United States, where mammoth conflicts arose over the North Atlantic Free Trade Area (Nafta) and where political activists including Donald Trump managed to scupper free trade agreements proposed for both America’s Pacific and Atlantic economic partners, there hasn’t been a serious fight over trade legislation in the House of Commons in the lifetime of all but the most ancient MPs.

Trade is handled by the European Commission because all the countries in a Customs Union abide by common trade rules. This has enormously helped Israel, for whom the EU is its third biggest trade partner.

Calls over the years for the UK to impose discriminatory trade barriers against Israeli goods and specifically those made by Israeli firms located in the West Bank have been easy to push away by ministers arguing that any trade measures have to be agreed at an EU level.

Countries like Germany and France, and many MEPs, effectively block anti-Israel trade measures despite the sustained campaign of boycott, disinvestment and sanctions which is growing in strength and noise as anti-Israel activists fuse with Islamist ideologues, especially on university campuses and in left-wing parties, to demand a full-throttle BDS policy.

For British ministers, whether Conservative or Labour, the fact the UK is in the EU Customs Union means the officials of the European Commission decide trade policy. This will change dramatically once the proposals revealed by Liam Fox, the International Trade Secretary, to create a new bureaucracy in London to decide what can and cannot be imported into Britain come into effect.

Barry Gardiner, Labour’s trade spokesperson, has repeatedly agreed with the government’s line that Britain must have its own trade policy.

Independently of different views on Brexit, politicians who seek to repatriate trade policy to make it domestic party politics should be careful what they wish for.

Dr Fox wants to set up a nine-strong “Trade Remedies Authority” to advise — not decide — on complaints from consumers, rival producers and presumably campaigners on animal rights or worker exploitation who will argue that trade in foie gras or anything made in Bangladesh or China avoids all international labour norms or is done by firms and governments that ignore human, LGBT or animal rights.

Israel will be the chief target once the UK has left the Customs Union. It is perfectly possible to “leave” the EU in the sense of formally quitting the EU Treaty without repatriating trade rules. Turkey for example is in a customs union with the EU, and Norway obeys all Single Market rules without being an EU member.

The BDS movement is growing in strength. In June it won a notable High Court victory, when Sir Ross Cranston, a former Labour minister and now a judge, ruled that the government proposal banning any boycott of Israeli goods was illegal.

If the UK does repatriate its trade policy it will be a field day for the BDS movement to launch campaigns at a constituency level to get MPs to back trade bans, especially on goods made by Israeli firms in the West Bank.

In his new book on Gaza, Donald Macintyre, the highly respected British political writer, says: “The EU could make a sharp impact by differentiating — but robustly — between Israel proper and Israeli settlements in the West Bank, including by a ban on imports of goods from the settlements”.

Behind the scenes the UK has slowed down or prevented EU moves against Israel. This will end when UK ministers no longer take part in deciding EU policy across a range of issues.

If Britain goes beyond leaving the EU as a political institution and moves to creating a national trade policy subject to decisions by MPs — rather than left to Brussels trade experts — the BDS movement will walk through this door opened by such a hard Brexit and mobilise massively to press MPs of all parties to use trade as a political weapon against Israel.

One does not have to be strongly pro- or anti-Brexit to believe that trade is best left where it is — in the hands of trade experts in Brussels rather than handed over to NGOs and the BDS movement.

Mrs May should thank Dr Fox for his work and quietly put his suggestion in the bottom drawer.

Denis MacShane is the former Europe Minister and author of Brexit, No Exit: (Why in the End) Britain Won’t Leave Europe (IB Tauris). He worked on international trade issues in Geneva before being elected MP for Rotherham. He is now a senior adviser at Avisa Partners, Brussels