"Megaphone diplomacy" will not get a good deal for Britain during Brexit, Jeremy Corbyn has warned the prime minister.
The Labour leader said threatening to walk away from the talks without an agreement was not a sensible way of dealing with countries responsible for half of the UK's overseas trade.
His comments came amid weekend reports that European Commission president Jean Claude Juncker said Mrs May was from a "different galaxy" after they met last week for dinner in Downing Street.
Campaigning in Battersea, south London, Mr Corbyn said a Labour government would approach the negotiations with "respect and sense".
"She (Mrs May) seems to be sending rather mixed messages.
"To start negotiations by threatening to walk away with no deal and set up a low tax economy on the shores of Europe is not a very sensible way of approaching people with whom half of our trade is done at the present time.
"Of course they are going to be difficult (negotiations), but you start from the basis that you want to reach an agreement, you start from the basis that you have quite a lot of shared interests and values.
"If you start from that basis and show respect, you are more likely to get a good deal. But if you start with a megaphone, calling people silly names, it is not a great start to anything."