As a new round of talks opens between Britain and the European Union, British Prime Minister Theresa May is under mounting pressure to take sides in the Conservative Party's war over Brexit.

May has walked a fine line between two feuding factions: those who want a clean break, even if it means trade barriers with Europe, and those who want to keep Britain's economy closely aligned to the 28-nation bloc.

With just months to negotiate future relations before Britain leaves in March 2019, the tensions are coming to a head. British Sunday newspapers teemed with allegations of plots against May by "hard Brexit" rivals including Foreign Secretary Boris Johnson.

The Sunday Times claims pro-Brexit lawmakers are prepared to topple May if she tries to keep Britain in the EU's tariff-free customs union.