The move by the Republican People's Party (CHP) comes after the president on Wednesday approved the bill, opening the way for a referendum on reforms opponents see as a threat to the Muslim country's secular system. AK denies Islamist ambitions and says the changes are gauged to meet EU entry criteria.

The secularist-oriented Constitutional Court has annulled other AK Party reforms in the past and reform of the court itself is central to the bill approved by parliament last week.

"The application to cancel the constitutional change will be made to the Constitutional Court tomorrow," the CHP source said.

There was no immediate reaction in financial markets to what was an expected announcement from the CHP.

Uncertainty over Turkey's politics rose earlier this week following the resignation of the CHP leader, the veteran Deniz Baykal, following the release of a videotape on the Internet purporting to show him and a woman in a bedroom.

Baykal accused the government of involvement in a conspiracy to bring him down.

The country's electoral commission is expected to announce soon when a referendum on the reforms will be held and is holding a meeting a 2 p.m. (1100 GMT), broadcaster NTV said.

AK Party officials said they expected the referendum to be held in mid-July.

"According to us the period for a referendum is 60 days. The day on which a referendum will be held is July 18, 2010," AK Party group deputy chairman Mustafa Elitas told reporters.

Critics are agreed that the constitution first drafted after a military coup in 1980 needs changing.

While there is little opposition to most of the proposed amendments, changes that would give the government greater say over the appointment of members of the senior judiciary are seen by critics as a move to consolidate the AK Party's power.

Critics believe the changes to the judiciary would violate the principle of separation of powers and strengthen the position of AK, whose roots lie in political Islam but which relies on broader support for its large parliamentary majority.

The bill reforms the structure of the Constitutional Court and an official body regulating judges and prosecutors and makes the powerful military answerable to civilian courts.

It would also render military officers responsible for the 1980 coup open to prosecution.

The opposition CHP, which has 97 deputies in parliament, will need 110 signatures from MPs in order to launch the court appeal to block the referendum.

Parliamentary support for the bill was less than the two-thirds majority needed to pass it directly into law, requiring that final approval be sought in a referendum.

The AK Party has said it is confident it will achieve enough support in the public vote.