Spanish Socialist Party leader Pedro Sánchez says EU-Turkey refugee deal is a "pact of shame." | Pablo Blazquez Dominguez/Getty Images Spain’s Sánchez wants EU-Turkey ‘pact of shame’ altered

The leader of Spain's opposition Socialists, Pedro Sánchez, asked acting Prime Minister Mariano Rajoy on Friday not to support the refugee deal between the European Union and Turkey that is due to be finalized at next week's summit.

The prospective deal between the EU and Turkey is "unfair" and unhelpful, said the Socialist (PSOE) leader, adding that there is still time to modify the agreement before the EU summit on March 17-18, when it is expected to be ratified.

"We have a week to change this agreement. The European Council of 17 and 18 March can not approve this pact of shame," said Sánchez, who failed last week in his bid to replace Rajoy as prime minister following December's inconclusive elections.

At this week's emergency EU-Turkey summit, Ankara effectively doubled the amount of EU aid it wants for looking after Syrian refugees to €6 billion. Turkey offered to take back economic migrants who land in Greece; in return, an equivalent number of Syrian refugees in Turkey will be distributed among EU members under a controversial quota system.

Spain's political stalemate since December's election is preventing the country from reaching consensus on EU issues, including the refugee crisis. Rajoy, whose Popular Party came first but lost its majority, refuses to appear in parliament to explain why he accepted the deal, which the majority of Spanish deputies — all parliamentary groups except the PP — reject.

A new poll by GESOP published in El Periódico on Friday suggests that if Spain has to hold new elections, the centrist party Ciudadanos, which came fourth in December, could increase its number of seats by 20 and come close to enabling a center-right majority with the PP. So far, Ciudadanos has supported Sánchez's failed attempt to form a new Spanish government.

Talks between parties will continue until May 2, but if no coalition deal is struck, parliament will be dissolved and new elections held on June 26.