The logo of Japanese car giant Nissan stands in the forecourt of a Nissan car dealership | Scott Heppell/ AFP via Getty Commission inquires about UK’s offer to Nissan

The European Commission has asked the United Kingdom for details about what assurances it gave Nissan that prompted the company to announce further U.K. investments despite fears Brexit could hamper its exports to Europe, a Commission spokesperson confirmed today.

Officials would like to check whether the controversial guarantees breach EU rules preventing governments from subsidizing favored companies or sectors, in a move that runs the risk of inflaming tensions ahead of negotiations over the U.K.'s exit from the EU.

The Japanese carmaker announced last week it would invest in building two new car models at its plant in Sunderland, in northeast England. The U.K. government has said it merely promised to seek a tariff-free trade deal with the EU, although detractors say it promised to subsidize losses ensuing from Brexit.

"We have seen press reports regarding this issue," said Ricardo Cardoso, a spokesperson for the Commission. "As a result, the Commission at services level is in contact with the U.K. authorities."

He added that such exchanges are common and that the Commission had not taken any formal view on whether the assurances amounted to state aid.

Under EU law, assurance and guarantees given to companies can amount to illegal state aid even if no money changes hands.

The interest by officials overseen by European Commissioner for Competition Margrethe Vestager constitutes a stark confirmation of Brussels' position that it will treat the U.K. as a member of the EU single market until it actually leaves.