The main rival to Turkish President Recep Tayyip Erdo?an faces a potential bar to running in elections after his university degree was revoked amid a mounting crackdown on the opposition.

Ekrem ?mamo?lu, the popular mayor of Istanbul who outperforms Erdo?an in opinion polls, was expecting to be selected as his party’s presidential nominee on Sunday.

But the cancellation by the state Istanbul University on Tuesday of his higher education degree, a constitutional requirement for the office, would disqualify him from entering the presidential race if it was not overturned on appeal.

It is the latest obstacle for ?mamo?lu, 53, who is facing a spate of criminal probes, up to 23 years in prison and a ban from politics. He accuses the government of “weaponising the judiciary” to maintain power.

“We clearly see that the foundation for legitimate, fair elections is being obstructed by the government,” ?mamo?lu said in a video posted to his X account. He said he would appeal against the decision but did not “have faith in a just ruling”.

Elections are scheduled for 2028, but Imamoglu and his party are holding a primary now in an attempt to pressure the government into calling a vote earlier.

Turkish authorities are enforcing a broad crackdown against critical voices, with arrests of journalists, businessmen and academics increasing in recent months. Erdo?an’s allies have said he should seek re-election after leading Turkey for more than two decades.

The Turkish leader cannot run for office again because of term limits, but he could extend his rule if constraints are abolished in a new constitution or if parliament calls an early election. He currently lacks enough support among lawmakers for either scenario.

?mamo?lu was re-elected by a margin of more than 11 percentage points last year amid a wave of voter discontent with Erdo?an’s ruling party and its handling of a long-running cost of living crisis. He has been mayor of the city of 16mn people since 2019.

The probe into ?mamo?lu’s diploma was opened by prosecutors the day after he announced he would seek the main opposition Republican People’s Party’s nomination for president.

Istanbul University said on Tuesday it was invalidating ?mamo?lu’s diploma in business administration, along with the degrees of 27 other people, due to irregularities stemming from their transfer to the school. ?mamo?lu had begun his university education in northern Cyprus.

He later received a masters degree, also from Istanbul University.

The state’s higher education board has far-reaching powers over universities in Turkey, and Erdo?an appoints the rectors of state institutions.

Berk Esen, a professor of political science at Sabanci University, said the move against ?mamo?lu could “backfire”. After building his reputation as a candidate “who can win”, ?mamo?lu might now simply broaden his appeal with the public as the “undisputed candidate” to lead the opposition, Esen said.

Erdo?an entered the national political stage as mayor of Istanbul in 1994 but was forced to step down four years later for reciting a poem that a court said “incited religious hatred”, and spent several months in prison.

Like Imamo?lu, Erdo?an has faced questions about the legitimacy of his degree because of claims that the faculty he attended became part of a full university a year after he graduated. Erdo?an has dismissed the allegations, and a copy of his diploma is publicly available.

He has also received several honorary degrees since taking office and has a university named after him in his ancestral province of Rize.