JAPAN has delivered a stark warning to Theresa May that she must deliver a soft Brexit or face seeing the country’s car manufacturers, banks and pharmaceutical companies relocate to the continent.

Tokyo’s shot across Britain’s bow came after US President Barack Obama made clear that, post the Brexit vote, a trade deal with the UK was not Washington's top priority; rather the bigger prizes of trans-Pacific partnership and the troubled US-EU trade deal were.

At the G20 summit in at Hangzhou in China, the Prime Minister insisted, despite the Brexit vote, Britain was “open for business” and would make a success of leaving the EU.

But Japanese government published a list of demands aimed at protecting the country's car manufacturers and financial institutions as well as jobs of Britons working for Japanese firms.

The 15-page ministry of foreign affairs document, based on requests from Japanese firms, urged the UK Government to behave in a "responsible manner".

It pointed out how many Japanese businesses had been invited by the UK Government to invest in Britain as a “gateway to Europe” and urged London to “consider this fact seriously and respond in a responsible manner to minimise any harmful effects on these businesses".

The report also warned head offices of Japanese firms in the UK could be shifted to the continent if EU regulations no longer applied after Brexit.

The document urged the negotiations between the UK and EU to be carried out as early as possible, stressing how it was imperative for them to “regain the confidence of the world and ensure their unwavering competitiveness by increasing the predictability of the Brexit process, ensuring the outcome is free of unpleasant surprises and reducing the risks emanating from uncertainty".

Key among the requests made by Japanese firms was maintaining "access to workers," who were nationals of the UK or EU; this could mean effectively keeping free movement.

Tokyo also called for the maintenance of current tariff rates and customs procedures and for the UK to keep the existing rules relating to financial services, including the "single passport" regime which allows firms in one member state to operate in another without seeking further authorisation.

Car manufacturers including Honda, Nissan and Toyota all have major manufacturing bases in the UK, employing thousands of workers, and the report warned Brexit could hit them hard by imposing extra levies.

Mrs May is expected to have a brief meeting with Japanese leader Shinzo Abe at the G20 summit on Monday.

A Number 10 official said: "Much as we want to take the time, as we prepare for negotiations, to understand where stakeholders in the UK are coming from, British businesses, different economic sectors, it is not unhelpful to have some of our trading partners setting out some of the issues that they are looking at."

But Tasmina Ahmed-Sheikh for the SNP said the Japanese Government document laid bare the “grim reality of Brexit for jobs and business in Scotland and sets out why we must do everything we can to protect Scotland's membership of the EU”.

The MP for Ochil and South Perthshire denounced the “incompetence” of the Tory government, which had overseen the Brexit vote, and added: “From the adverse impact on Scotland's financial services sector if the passporting issues are not resolved, to the added costs and inflated bureaucracy that would flow from being outside the customs union, our economy shouldn't be seen as expendable to achieve Liam Fox's isolationist fantasy."

Meantime in what could be one of the most tense bilaterals, Mrs May is due to meet Chinese President Xi Jinping after the G20 summit ends on Monday to discuss the £18 billion Hinkley Point nuclear power station plan in Somerset. The state-owned China General Nuclear is due to take a one-third stake in the project.

The PM defended the delay on whether to give it the go-ahead, saying it was down to "the way I operate," noting: “I actually look at the evidence, take the advice, consider it properly and then come to a decision.” This is due later this month.