Turkey's secularist president has blocked a law that he says could have given Islamic extremists more freedom.
Turkey's president wants to deter 'separatist' teaching of the Koran
President Ahmet Necdet Sezer said the change could have encouraged Islamists to preach against Turkey's secular state in illegal hardline schools.
The law was a late addition to a new penal code which Turkey has adopted in its efforts to join the EU.
It was backed by the governing AK Party, which has Islamist roots but insists it is committed to secularism.
The proposed change would have cut the penalty for anyone found guilty of teaching the Koran in unauthorised centres.
At the moment, those found guilty can be jailed for between three and six years.
Under the change, that sentence would have been cut to between three and 12 month.
Mr Sezer argued that the new law therefore almost supported the opening and running of the illegal centres.
"It is obvious that the country's unity in the future will be endangered by the perverted mindsets of citizens educated in illegal separatist and religious educational institutions," Mr Sezer said in a statement.
The new penal code, which is a condition for Turkey starting European Union entry talks in October, came into effect on Wednesday. It will stay in force unchanged while the row is sorted out.
EU diplomats say they will monitor carefully the implementation of the wider code, but have dismissed the Koran courses issue as a Turkish domestic problem.
Parliament will now have to reconsider the change which Mr Sezer blocked.
Parliament can overturn the veto by approving it a second time.
But Mr Sezer could still challenge the legislation via the Constitutional Court.