"The working group, set up in the presidential secretariat, which deals with the constitutional reform issue, is improving the current wording of the Constitution," Roman Zvarych said.
Zvarych is a member of Yushchenko's Western-leaning Our Ukraine party, which announced in October that it was going into opposition amid a growing rift between the president and the governing coalition, led by Prime Minister Viktor Yanukovych's pro-Russian Party of Regions.
He said the working group is highly professional and mainly comprises former Constitutional Court judges.
As of January 1, 2006, Ukraine shifted from a presidential-parliamentary to a parliamentary-presidential form of governance, after amendments adopted by the Supreme Rada in December 2004 entered into force. As a result, some presidential powers were transferred to the country's parliament and Cabinet.
In late November, Yushchenko said the Constitution needed improving.
"I am convinced that the president's powers should be enhanced. They demand a greater balance, especially in relations with the government," he told three leading Ukrainian television channels.
Yulia Tymoshenko, the leader of an eponymous opposition bloc in parliament, said: "Constitutional reform led to a political crisis, and I don't see any way out but to cancel political reform and conduct early parliamentary elections."
