Turkey's two main secular opposition parties on Sunday formally backed a devout Muslim to challenge an expected bid for the presidency by the country's prime minister Recep Tayyip Erdogan, who heads an Islamic-rooted party.

The Republican People's Party (CHP) and Nationalist Movement Party (MHP) submitted a joint application to parliament to nominate Ekmeleddin Ihsanoglu, an academic and career diplomat, as their candidate in August presidential elections.

In a televised meeting with Parliament Speaker Cemil Cicek, senior opposition lawmakers submitted the signatures making Ihsanoglu the first candidate to formally enter the presidential race.

Erdogan, whose ruling Justice and Development Party (AKP) traces its roots to political Islam, is widely expected to be unveiled on Tuesday as his party's candidate for Turkey's first democratic presidential vote on August 10.

The secular opposition delivered a big surprise earlier this month by throwing their support behind Ihsanoglu, a little-known devout Muslim tasked with winning votes from the AKP's traditionally pious electorate.

The decision to back a political novice, seen by many as conservative and Islamic-leaning, alarmed the secular segments of the society, who accuse Erdogan of forcing Islamic values on the predominantly Muslim country.

But Ihsanoglu clearly stressed the need to "keep religion out of politics".

Twenty-one lawmakers from the CHP refused to sign their names while ultra-nationalist MHP's lawmakers gave Ihsanoglu their full backing.

Born in Cairo to Turkish parents, 70-year-old Ihsanoglu stepped down in December as head of the Organisation of Islamic Cooperation (OIC) and served as an envoy to Bosnia and Gambia in a long diplomatic career.

Speaking five languages, he is seen as a reconciliatory and moderate figure, in stark contrast to Erdogan, whose uncompromising stance critics say has left Turkish society more polarised than ever.

But Erdogan, whose AKP has won every election since 2002, remains the most popular leader in the country, with polls suggesting that he will win an outright victory in the first round on August 10.

A recent survey by pollster Genar gave Erdogan 55.2 percent of the vote against Ihsanoglu on 35.8 percent.