Ministers have warned that the UK could take longer to leave the EU than expected, even as May prepares to trigger Article 50 of the Lisbon Treaty, which would begin the process.

The prime minister has been expected to begin the negotiations in early 2017. However, the Sunday Times reported that some within the government are suggesting May might wait until autumn of that year to start the two-year process, effectively pushing back the UK's departure from the EU until late 2019.

"They don't have the infrastructure for the people they need to hire. They say they don't even know the right questions to ask when they finally begin bargaining with Europe," one source in London told the newspaper.

Another source said Brexit negotiations were further complicated by the fact that Germany and France will be holding major elections next year.

France is slated to hold presidential election rounds in April and May, while Germany is due to hold its general election in the fall.

Meanwhile, the minister in charge of negotiations, David Davis, has reportedly hired only a fraction of the staff needed to fill a department devoted to carrying out the Brexit, which was created under May, Deutsche Welle reports.