* Gul's term as head of state to end in August
* Erdogan expected to run for president
* Next prime minister set to be close Erdogan loyalist (Adds details and quotes)
By Orhan Coskun and Humeyra Pamuk
ANKARA, April 18 (Reuters) - Turkey's president appeared to rule out a job swap with Prime Minister Tayyip Erdogan when his term as head of state ends in a few months, signalling strains between the allies following months of political tension.
President Abdullah Gul's comments on Friday threw open the question of who might succeed Erdogan if he runs for president in an August election as expected, and raised the prospect of him picking a close loyalist to cement his grip on power.
The president has until now been chosen by parliament and played a largely ceremonial role, but August's election will be the first direct vote for the post. Erdogan has said that will give the presidency more authority, and has vowed to exercise its full powers if elected.
Gul co-founded the ruling Islamist-rooted AK Party with Erdogan and had been seen as a potential future prime minister should Erdogan, who has dominated Turkish politics for more than a decade, become head of state.
A more conciliatory figure than the prime minister, Gul has been seen as a check on Erdogan's authoritarian impulses, although their relations have grown increasingly strained.
"I don't have any political plan for the future under today's conditions," Gul told reporters in the western province of Kutahya, when asked about the presidential election.
Asked if a "Putin-Medvedev model" was conceivable, Gul said such a formula would not be "appropriate" for a democracy but did not elaborate.
Russia's current Prime Minister Dmitry Medvedev replaced Vladimir Putin as president in 2008, while Putin became prime minister. They swapped roles in 2012.
Erdogan told reporters after Friday prayers he had not heard Gul's words and would not comment before speaking with him.
A majority of deputies in the AK Party voted in a secret ballot on Wednesday in favour of an Erdogan presidential bid.
Senior party officials have said Gul would be highly unlikely to try to run against him, a possibility the president also seemed to rule out on Friday.
But Erdogan, who has failed to push through the constitutional changes he wanted to create an executive presidency in Turkey, could also seek a fourth term as prime minister if the AK Party changed its internal rules on three-term limits. Gul could then remain as president.
"If Erdogan doesn't give up on going (for the presidency), Gul won't go against him. But not going against him doesn't mean he would let himself be crushed politically," said Eyup Can, editor of the liberal daily Radikal.
"Gul is not saying farewell to politics. On the contrary, he is conducting 'hardcore politics'," he said, seeing Gul's words as a warning to Erdogan not to force his presidential ambitions.
While Erdogan and Gul have been close allies during their political careers, they have appeared to fall out on occasions.
Last month, Gul openly contradicted the prime minister by dismissing suggestions that outside forces were conspiring against Turkey in a corruption scandal which has shaken Erdogan's government since last December.
Gul was also at odds with Erdogan over bans on social media sites such as Twitter, Facebook and YouTube , after the premier suggested they should be shut to stop a stream of leaked recordings fuelling the scandal.
Twitter was blocked by the authorities for two weeks, while access to YouTube remains blocked.
Erdogan had long wanted to change the constitution and create an executive presidency, but political opposition to such a move has checked those plans for now.
Gul's absence could pave the way for a more malleable figure to assume the post of prime minister should Erdogan win the presidency, enabling him to shift the balance of power towards the head of state even without a full presidential system.
AK Party Deputy Chairman Mehmet Ali Sahin, seen as staunch Erdogan loyalist and a fellow party founder, has been mooted as the most likely candidate.
In the longer term, Erdogan looks unlikely to abandon his ambition to create a full presidential system.
He confirmed on Friday that the AK Party is preparing a draft law to change the nationwide electoral system, a move which could increase its majority in parliament and make it easier to drive through constitutional changes.
Senior AK Party officials said the proposed system would eliminate the current 10 percent threshold of votes needed for a party to enter parliament, among the highest in the world, and redefine electoral boundaries. (Reporting by Orhan Coskun and Humeyra Pamuk; Writing by Daren Butler; Editing by Nick Tattersall and Andrew Roche)