Nissan will conduct a review of its Sunderland plant’s competitiveness once the UK’s future relationship with the EU is settled, the head of the Japanese carmaker has said.

Carlos Ghosn’s remarks on the sidelines of the World Economic Forum in Davos may raise concerns about the commitment of company to the UK, following its pledge in October to turn the factory into one of the biggest car plants in the world.

The government has faced questions about the “support and assurances” given to Nissan before the company’s October’s announcement that new versions of its Qashqai and X-Trail would be made in the north-east of England, turning Sunderland into a “super plant” producing more than 600,000 cars a year.

Ghosn was reported as saying that Nissan trusted Theresa May’s assurances, but that the company would need to review the situation. “Obviously when the package comes, you are going to have to re-evaluate the situation, and say, ‘OK, is the competitiveness of your plant preserved or not?’

“We are going to have to make decisions on investment within the next two to three years, so obviously the faster the Brexit results come, the better it is,” Reuters quoted Ghosn as saying.

The government has refused to publish the letter to Nissan in which assurances were given by Greg Clark, the business secretary.

May, who set out her vision for a post-Brexit Britain in a speech on Tuesday, was in Davos to meet bankers and business leaders. In a keynote address she urged delegates to address the effects of globalisation.

A senior Toyota executive at the Swiss ski resort told the Financial Times that the carmaker’s UK plants also needed to be competitive after Brexit. Takeshi Uchiyamada, Toyota’s chair, said: “We have seen the direction of the prime minister of the UK, [so] we are now going to consider, together with the suppliers, how our company can survive.”