Japanese carmaker Nissan and Royal Bank of Scotland have became the latest companies to warn about damage to the economy if Britain fails to secure a trade deal before leaving the European Union. 

Less than six months before Britain is due to exit the bloc, Prime Minister Theresa May has yet to find a proposal for economic ties that pleases EU negotiators and both sides of her divided Conservative party. 

The UK government has stepped up planning for a no-deal Brexit when the world's fifth-largest economy leaves the EU on March 29, 2019.

This step could spook financial markets and dislocate trade flows across Europe and beyond. 

Nissan, which operates the UK's largest automotive factory, said leaving without a deal would have "serious implications" for Britain's manufacturing industry. 

In the two years since the Brexit vote, Nissan has been restrained in public about the referendum. But its warning now shows the growing alarm among business executives. 

"Today we are among those companies with major investments in the UK who are still waiting for clarity on what the future trading relationship between the UK and the EU will look like," Nissan said in a statement. 

"We urge UK and EU negotiators to work collaboratively towards an orderly balanced Brexit that will continue to encourage mutually beneficial trade," the company said.

Royal Bank of Scotland's chief executive Ross McEwan said Britain's economy might fall into recession without a deal. 

Ross McEwan said companies were scaling back investment due to the uncertainty and RBS was more cautious about lending to the retail and construction industries in particular as Brexit approached. 

Royal Bank of Scotland CEO Ross McEwan

"Big businesses are pausing, they are saying that in six months time I'll have another look at the UK and I might come back, but if it's really bad I'll invest elsewhere - that's the reality of where we are today," McEwan told the BBC. 

If Britain fails to agree a deal with the EU then the country would move from seamless trade with the world's largest trading bloc to customs arrangements set by the World Trade Organisation for external states with no preferential deals. 

Britain's Society of Motor Manufacturers and Traders said last month that tariffs of 10% under WTO rules would add an average of €3,000 to the cost of British-built cars sold in the EU if fully passed on to buyers. 

Car makers are worried that port and road delays could slow the movement of finished cars and parts, crippling output and adding costs, if Britain fails to reach an agreement. 

Nissan said in 2016 it would build its next generation Qashqai SUV and a new X-Trail model at its Sunderland plant in England, a major boost to the government just a few months after Britain voted to leave the EU. 

A source told Reuters that the company had received a letter from the government promising Nissan extra support if Britain's departure from the EU hit the competitiveness of the plant. 

Japan's ambassador to the Britain, Koji Tsuruoka, also issued a blunt warning to the government after meeting British ministers in February. 

"If there is no profitability of continuing operation in the UK - not Japanese only - no private company can continue operation. It's as simple as that," he said.