Poland, which lost a diplomatic campaign to oust its former premier Donald Tusk from his post as European Council president, has now accused the EU of "cheating" and announced a "negative" policy towards Brussels.
"It turned out that EU policy is one of double standards and cheating", Foreign Minister Witold Waszczykowski in the weekend edition of the Super Express newspaper.
Waszczykowski was referring to last week's clash at a Brussels summit of EU leaders, when Poland cast the sole vote against Tusk's re-election.
Warsaw's far-right and eurosceptic government had campaigned hard against the centrist Tusk, 59, who was prime minister from 2007 to 2014 and in the top EU job since then, arguing he should not be elected against the wishes of his home country.
But bloc leaders voted by 27 to one to give Tusk a fresh two-and-a-half-year mandate -- with only Poland's current Prime Minister Beata Szydlo voting against.
"We must be conscious that at any moment we may be deceived", Waszczykowski wrote.
"We must adapt our policies and adjust our behaviour concerning the EU. Certainly, we must lower our level of confidence in the EU. We will have to start following a negative policy", he continued, saying that could include "blocking" various European initiatives.
The minister pointed to the refusal of the European Council to consider the last-minute alternate candidate Warsaw had put forward to replace Tusk -- euro-MP Jacek Saryusz-Wolski.
"Why was it decided that the candidacy of eurodeputy Jacek Saryusz Wolski had to be decided by a consensus vote? And that for Donald Tusk, it was going to be a simple majority vote?" he said.
But Waszczykowski said Poland's need for a "very firm" stance on the EU did not mean the country would balk on obligations to participate in the European Council of bloc leaders and EU activities in general.
But the diplomatic clash underscored a deepening rift with the largest of the eastern post-Soviet EU states, just weeks before the bloc had hoped to make a show of unity at its 60th birthday and as Britain moves forward with plans to exit the EU.
Moreover, the clash rattled an alliance struck between ex-communist states Poland and Hungary, which are both led by eurosceptic rightwingers.
Hungarian Prime Minister Viktor Orban, who had initially supported the Polish stance, ended up siding with the rest of the EU, leaving Szydlo out in the cold.
After the vote, Jaroslaw Kaczynski, the powerful head of Poland's ruling Law and Justice (PiS )party, voiced disappointment with Orban. Poland's deputy foreign minister postponed a trip to Budapest planned the day after the vote.
But Warsaw said both sides would pursue their cooperation, including a visit by Hungary's President Janos Ader to Warsaw later in March to celebrate "days of Polish-Hungarian friendship".
"I am sure Presidents Andrzej Duda and Janos Ader will speak about Europe and what happened in Brussels", Marek Magierowski, spokesman for the Polish presidency, said.