World politicians warned Theresa May of the potential cost of Brexit to jobs, investment and trade as they urged her to end uncertainty over the deal she will seek.

As the G20 summit closed in China, the Italian government joined Japan in laying bare the consequences of the Prime Minister’s stated determination to curtail free movement of EU citizens.

Carlo Calenda, Italy’s economic development minister, struck an aggressive tone, telling the UK to hurry up its departure and also be ready to face curbs on exports to the EU when it decides exactly how it will exit the bloc.

The comments came after Japan’s ambassador to the UK warned that Japanese companies will consider pulling operations out of the UK if their “requests are not met” in the Brexit deal.

But Ms May continued to strike a confident stance about the UK’s post-Brexit future, naming a string of countries eager to start talks aimed at removing barriers to trade.

Mr Calenda, interviewed in Italy by Bloomberg Television, stressed the need for the remaining EU countries to speed up progress on migration and trade, without the distraction of the UK talks.

He said: “We cannot waste two years by negotiating with the UK how to maintain them inside without them wanting to be inside. We cannot afford this paradox.

“The more they are going to regulate and limit the presence of EU citizens in the UK the more we are going to limit the presence of UK goods into Europe.

“There should be a balance there. You cannot be here in the single market but at the same time excluding some part of the European relationship, which is very important.”

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Meanwhile, Koji Tsuruoka further stoked fears about the prospects for 140,000 people employed by Japanese firms – including giants such as Nissan, Hitachi, Fujitsu – if access to the vital EU market is lost.

The ambassador stressed that Japan wants Brexit to be “a success” and that its business leaders “all agree the UK is the best place to do business in Europe” – at present.

But, asked if they could pull out if Brexit goes wrong, he warned: “That’s an open question yet to be answered, because we do not know – of course, all options are available to the private sector.

“Their duty is to produce profits – if the way Brexit ends up does not provide companies with a prospect of making sufficient profits to continue operating in the UK, of course there’s no option they cannot choose. All options are open.”

Theresa May reiterates intention to leave the EU at G20

At the weekend, Japan issued a memo listing demands, including continued trade with no customs duties and harmonised regulations and standards between the UK and the EU.

Speaking to BBC Radio 4, Mr Tsuruoka added: “There is a negotiation that will have to be conducted. If there are variations, if those requests are not met, it will be up to the industry to decide what to do.”

In China, Ms May said: “As the UK leaves the EU, I've set out our ambition to become the global leader in free trade. In my bilateral meetings I’ve signalled our determination to secure trade deals with countries from around the world.

“The leaders from India, Mexico, South Korea and Singapore said that they would welcome talks to removing barriers to trade in our countries, and the Australian trade minister will visit the UK this week to take part on exploratory discussions on the shape of a UK Australia trade deal.