Ivan Rogers, Britain's envoy to the EU, warned ministers that the European consensus was that a deal might not be done until the early to mid-2020s and that national parliaments could ultimately reject it, the BBC said.
PM May's office said it did not recognize the views expressed by the diplomat, the BBC said.
According to Reuters, May has said she will invoke Article 50, the two-year divorce process for leaving the EU, by the end of March.