Turkey's top court on Thursday annulled a law allowing women to wear Islamic headscarves in universities on grounds it violates secularism, in a major blow for the prime minister and his Islamist-rooted party.

In a brief statement after a seven-hour session, the 11-judge tribunal said it scrapped the law because it ran counter to constitutional provisions which say Turkey is a secular republic and that this principle is unalterable.

The constitutional amendment, pushed through by the ruling Justice and Development Party (AKP), was cited by the country's chief prosecutor as a key piece of evidence in his pending bid to outlaw the party on charges that it is covertly seeking to replace the secular order with an Islamist regime.

The ruling is largely seen as an indication that the Constitutional Court will also go against the AKP when it rules on whether to ban it and bar 71 party officials, among them Prime Minister Recep Tayyip Erdogan and President Abdullah Gul, from politics.

"It is a historic ruling... It has demonstrated that secularism is Turkey's state ideology," veteran politician and jurist Husamettin Cindoruk said. "It signals that hard times are coming for the AKP," he added.

A senior party member denounced the ruling on Thursday's, saying that the court had overstepped its jurisdiction, limited to examining only whether constitutional amendments are procedurally flawed.

"This is interfering with both democracy and parliament's legislative authority," Bekir Bozdag said. "The parliament's power to amend the constitution has now become subject to the approval of the Constitutional Court."

The AKP, the moderate offshoot of a banned Islamist movement, pushed the headscarf amendment through parliament in February despite fierce objections that the change was a threat to the strict separation of state and religion.

It argued that the headscarf ban - imposed after a 1980 military coup - violates freedom of conscience and the right to education, but the main opposition party immediately asked the Constitutional Court to abolish the law.

The court has in the past twice ruled against moves to lift the on-campus ban on the headscarf.

The ban has also been upheld by the Council of State, Turkey's top administrative court, as well as the European Court of Human Rights.

Hardline secularists - among them the army, the judiciary and academics - see the headscarf as a symbol of defiance against secularism, a basic tenet of the 84-year-old republic.

They say that easing the restriction in universities will increase social pressure on women to cover up and pave the way for the lifting of a similar ban in high schools and government offices.

The AKP rejects charges of being anti-secular but has been under attack from the secularist camp ever since it first came to power in 2002. The party was re-elected for a second term in July with nearly 47 percent of the vote drawing mainly on its economic success.

It says it has disowned its Islamist roots and embraced Turkey's bid to join the European Union, but it also maintains that rigid interpretations of secularism in Turkey breach religious freedoms.

Opponents argue that moves such as allowing the Islamic headscarf in universities and banning alcohol sales in restaurants run by AKP municipalities, coupled with rhetoric in favour of broader religious freedoms, indicate a secret Islamist agenda.

The Constitutional Court is expected to rule on the bid to ban the AKP late this year.