David Cameron will continue his diplomatic tour of Europe as part of the bid to renegotiate Britain's relationship with Brussels.
The Prime Minister will meet Bulgarian counterpart Boyko Borissov and president Rosen Plevneliev for talks in capital Sofia to push the case for reform.
It comes ahead of a crucial meeting of EU leaders later this month that Mr Cameron hopes will result in a deal, although president of the European Council Donald Tusk has said that will be "very difficult" to achieve.
The Prime Minister has submitted a series of demands for changes in the relationship with the EU ahead of the in/out referendum promised by the end of 2017.
A quick agreement will allow him to make the case for staying in and give him more flexibility in choosing the date for the public vote.
But if European leaders fail to agree on an acceptable package of reforms, the Prime Minister has insisted he will not rule out recommending a vote to quit the EU.
Mr Cameron will also discuss pressures on EU borders amid the continuing migration crisis in today's talks.