Theresa May’s plan to sign companies up to European Union regulators to maintain their access to markets after Brexit was cast into doubt as a senior EU official expressed reservations.

May proposed that the U.K. could remain part of EU agencies that regulate the medicine, chemicals and aviation industries to keep close to the bloc after Brexit in areas where it suits the U.K. economy.

Stefaan De Rynck, an adviser to EU chief negotiator Michel Barnier, indicated late on Monday that the plan raised potential conflicts. The agencies enact the decisions of the European Commission, they “operate in a context where single market principles operate,” and while their work is technical, “it can become political,” he said. Only three non-EU countries now sign up to the regulators—Norway, Iceland and Lichtenstein.

De Rynck was also cool on the prospect of mutual recognition for finance, underlining the importance of financial stability and the risks posed by integration without common oversight.

The EU has yet to respond to May’s Mansion House speech, but has made clear that a partial approach to the single market isn’t acceptable. The EU is due to set out its negotiating position on the future trading relationship this week.