Poland's right-wing prime minister branded a EU migration reform package a diktat from Brussels and Berlin and pledged a "firm veto".

European Union member states on Wednesday agreed to the final part of an overhaul for rules on how they handle asylum seekers and irregular migrants.

The issue is set to figure prominently at two days of summits involving EU leaders in Spanish city Granada starting Thursday.

Once implemented, the new Pact on Migration and Asylum would seek to relieve the pressure on so-called frontline countries such as Italy and Greece by relocating some arrivals to other EU states.

Those countries opposed to hosting asylum-seekers would be required to pay the ones that do take migrants in.

"A very clear question should be asked: why should we agree to a diktat from Brussels, a diktat from Berlin?" Mateusz Morawiecki said in a statement before leaving for Spain.

"I'm going to Granada for the summit of the European Council to put a firm veto," Morawiecki said.

Warsaw's opposition cannot derail the deal as it only needs a weighted majority of countries to be implemented.

"We don't agree with this diktat, because not only it doesn't solve the problem -- it creates a bigger problem and means inviting to Europe hundreds of boats with illegal economic migrants," Morawiecki said.

"We don't want another Lampedusa in Poland," he said, referring to the southern Italian island where thousands of migrants have landed after making a perilous crossing across the Mediterranean.

Anti-immigrant rhetoric has for years been a staple of the governing conservatives in Poland which is gearing up for a general election next month.

The populists in power have also scheduled a national referendum on the EU migration reform to be held on the day of the vote.

The opposition has called for boycott over the "biased" question on the ballot.

"Are you in favour of accepting thousands of illegal immigrants from the Middle East and Africa, in line with mandatory relocation scheme imposed by the European bureaucracy?" is what people will be asked.