A Downing Street spokesperson said:
In a trilateral meeting at the Western Balkans Summit in Sofia, the Prime Minister discussed the Brexit negotiations with President Juncker and President Tusk.
The leaders discussed the positive progress which had been made in the talks so far, including agreements on citizens rights and other matters.
The Prime Minister reiterated the UKs commitment to the joint report agreed in December.
She said the proposal put forward by the European Commission in relation to the Ireland/Northern Ireland border was unacceptable. The PM said the UK would be shortly putting forward its own backstop proposal in relation to customs.
The leaders spoke of the need for a strong security partnership between the UK and the EU post-Brexit.
The Prime Minister also held a bilateral meeting with Taoiseach Leo Varadkar.
They agreed on their shared commitment to avoid a hard border between Ireland and Northern Ireland and on the need to c