Theresa May has been urged to keep trade as free as possible to promote economic growth in the decades to come, as she enters Brexit negotiations on Monday.

If the world turns its back on free trade and plunges into protectionism, the UK’s economy would be 3.7pc smaller in 2035 than it would otherwise be, analysts at Zurich have warned.

That is a shortfall of around $140bn, (£109.5bn) according to its estimates.

By contrast if the world embraces a radical new round of free trade and globalisation, Zurich believes it would add another 3.6pc to the UK economy in the coming 20 years, equivalent to adding another $140bn to our national income.