BRUSSELS (Reuters) - The European Commission called on Turkey on Tuesday to investigate alleged irregularities in Sunday’s referendum boosting the powers of President Tayyip Erdogan and urged him to show restraint after his narrow victory.

Declining to congratulate the Turkish president, the EU executive’s second official response since the vote instead focused on observer findings that the vote was skewed in Erdogan’s favour without a proper legal framework and with late changes in ballot counting.

“We call on the authorities to launch a transparent investigation into these alleged irregularities,” Commission spokesman Margaritis Schinas said when asked about the conclusions of the observer mission from the Council of Europe human rights body.

Turkey’s High Electoral Board made a last-minute decision on Sunday to count ballots that had not been stamped by officials.

The European Union opposed Erdogan’s bid to shift the country to a system giving the president sweeping new powers. The narrow 51.4 percent margin of victory also revealed doubts among Turkish voters about the constitutional changes.

Unlike U.S. President Donald Trump, the European Union would not congratulate Erdogan on his triumph, a Western official with knowledge of EU policy told Reuters.

“There will be no call to Erdogan from the Commission, certainly not a congratulatory call,” the official said. “Turkey is sliding towards a semi-authoritarian system under one man rule.”

The EU’s focus now is to decide the future of Turkey’s EU accession negotiations, the official said. A presidential system with few checks and balances is unlikely to meet the democratic credentials a country needs to join the bloc.

EU foreign ministers will meet on April 28 in Malta to discuss the next steps. A rapid implementation of constitutional reforms by Erdogan, taking full control of his political party and nominating senior judges, would likely strengthen the hand of countries willing to formally suspend accession talks, the official said.

Turkey, a NATO member state that began talks to join the EU in 2005, remains a crucial partner for the bloc by taking in millions of refugees fleeing from the six-year-old war in Syria.

But even before the referendum, a crackdown by Erdogan since a failed coup last July has alienated the bloc that prides itself on human rights and the rule of law. Relations have further soured since Erdogan accused the German and Dutch governments of acting like Nazis after they banned referendum campaign rallies by Turkish officials.

The European Union’s offer of a revamped, broader trade pact with Ankara could still act as leverage to slow Erdogan’s ambitions, however, a second Western official said.

The Commission said that any legislation bringing back the death penalty to Turkey, as pledged by Erdogan during his campaign, would certainly end Ankara’s EU membership bid.

“On the death penalty...not only is this a red line, but the reddest of all red lines,” Commission spokesman Schinas said. “We have an unequivocal rejection of the death penalty.”