Warsaw and Budapest have vetoed the bloc's 1.8 trillion euro financial package for 2021-2027 because it links access to the money with an obligation to respect the rule of law. Both capitals are under EU investigation for not respecting it.
Earlier on Thursday, Polish and Hungarian prime ministers jointly repeated their objection to the rule of law condition, digging in their heels as the bloc seeks to unwind the deadlock.
"It is clear that there is absolutely no support for reopening the conditionality mechanism in the European Parliament or in the Council," the senior diplomat said.
"With their statement Poland and Hungary are moving deeper and deeper into isolation," the diplomat said.
(Reporting by Jan Strupczewski)