Mrs May is taking Oliver Robbins, the civil servant leading the Brexit talks, with her for the dinner - the first since a frosty meal in Downing Street in April. Mr Juncker was reported to have launched a scathing attack on Mrs May during the April dinner, saying that Brexit “cannot be a success".
Downing Street sources insisted Monday's meeting had "been in the diary for weeks" but the announcement caused surprise in Westminster, and comes after last week's negotiations ended with little movement.
Mr Barnier said after the fifth round of discussions had ended without making any "great steps forward" and there was "disturbing deadlock" over the size of Britain's divorce bill.
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