As Jeremy Corbyn has argued over Brexit, Labour must prioritise jobs and the economy, and that, for 50 Labour parliamentarians who have signed a new statement, means staying in the single market and the customs union – something the TUC and business also want.

It’s the largest, richest market in the world: with more than 500 million people, and worth £11tn – fully a quarter of global GDP. It accounts for half our trade, and a surplus in services of £17bn.

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Over the 10 or so years that it will take to adjust to the shock of exiting the EU, including its single market, the British economy will end up 6% smaller in 2030 than it would have been if we had stayed, according to the Treasury – a massive hit on jobs and prosperity.

When the country voted to leave the EU by a narrow margin, the single market was not on the ballot paper. It almost never came up on the doorstep. Some leave leaders such as Boris Johnson said we should stay in it, others not.

Leaving would need to be followed by subsequent trade agreements – and not only with the remaining EU and European Economic Area member states, but also new agreements under World Trade Organisation rules with about 52 “third countries” outside the EU with which we have existing trade deals at present through the EU.

These will take many years to negotiate: Canada’s with the EU took seven years, and covered only manufactured goods. That’s no good for the UK, which is an 80% service economy.

Free-trade agreements do not normally cover many services, because of the complexity involved. Existing single market legislation covers: authorisation mechanisms and systems for granting licences to provide services; systems of mutual recognition of licensing requirements; commitment to common monitoring for observing standards; and a dispute resolution mechanism.

Does the single market mean uncontrolled EU migration? No.

Unless we stayed in the single market, any newly negotiated UK access agreement would have to be ratified by all 27 member states across more than 35 parliaments, including regional ones such as that of Wallonia in Belgium, which tried to hold up the ratification of the Canadian deal.

The removal of non-tariff barriers with the EU (especially standards for goods and services) would be tough to negotiate, because they are numerous, complex and subject to frequent updating through amendments. EU standards for cars alone cover thousands of pages of legislation.

This interim between leaving and finalising free trade deals could take years, with UK companies then facing trade barriers such as customs duties; customs procedures such as country of origin checks at borders; quotas and other quantitative restrictions; and other non-tariff barriers, including standards for goods and services.

The EU “passport” of regulatory equivalency that EU institutions currently grant to UK banks and other financial institutions will no longer apply if we leave the single market – a huge threat to a sector estimated to provide 11% of Treasury revenues and 10% of GDP. Already leading banks plan to move jobs to rival financial centres such as Frankfurt, Dublin, Amsterdam or Paris.

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Furthermore, single market rules come with the social, environmental, health and safety standards and workers’ and consumers’ rights that we all take for granted. Hard-right Brexiteers see an opportunity to water down this “red tape”, which they find an anathema.

Does the single market mean uncontrolled EU migration? No. The right to free movement has never been unconditional; Belgium, for example, imposes restrictions by returning to their EU country of origin each year thousands of migrants who do not have jobs.

In line with our September 2016 annual conference policy, Labour should lead the drive to remain in the single market and the customs union. There is probably now a cross-party parliamentary majority for that – certainly for a multi-year transitional deal that could even become permanent.

• A version of this article appeared on Progress Online