ISTANBUL (REUTERS) - Turkish architects have urged Pope Francis not to set foot in President Tayyip Erdogan's controversial new 1,000-room palace when he visits Ankara this month and legitimise what they deem an illegal building.

The sprawling palace, inaugurated last month and set to cost more than half a billion dollars, is constructed on land bequeathed to the state as a forest farm by Mustafa Kemal Ataturk, the founder of the modern Turkish republic.

Critics have decried its huge columned structure, atriums and marble floors as a symbol of Erdogan's extravagance and autocratic nature just three months after he took office as Turkey's first popularly-elected head of state.

"The architects sent a letter to Pope Francis, who will be the first guest in this illegal palace... They asked the pope not to honour an invitation to this building," the Ankara branch of the Turkish Chamber of Architects said on its website.

Pope Francis will visit Ankara and Istanbul on Nov 28-30.

Several court orders blocking the project - nicknamed Ak Saray or the White Palace - failed to halt construction, infuriating environmentalists who said it despoiled one of Ankara's few remaining green spaces.

Previous presidents used Ataturk's more modest old palace, but the decision to move comes as Erdogan launches what he has dubbed a "New Turkey".

He has made no secret of his ambition to amend the constitution and create a full executive presidential system.