David Cameron held a 10-minute telephone call with Israeli leader Benjamin Netanyahu to discuss the latest developments in the talks, Downing Street said, without giving details of their conversation.
This came after the White House said US President Barack Obama called the Israeli leader on Friday to assure him of America's continued commitment to preventing Iran from obtaining a nuclear weapon.
It appears increasingly likely that the talks will end without agreement, with the French Foreign Minister Laurent Fabius telling France-inter radio "differences" remained and that negotiations would resume in a week to 10 days if no agreement was found by the end of the day.
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Theresa May will gather Cabinet ministers together on Wednesday to discuss her plans for Brexit.