Spanish MPs on Friday rejected acting Prime Minister Mariano Rajoy's bid to form a minority government for the second time in 48 hours, setting the country on course for a possible third election in a year.

Rajoy, leader of the conservative Popular Party, lost the vote of confidence with 180 votes against and 170 in favour -- the same result he obtained during the first vote on Wednesday.

He was backed only by members of his own group, by centrist upstarts Ciudadanos and a lone member from a regional party from the Canary Islands.

Political leaders now have two months from Wednesday's parliamentary vote to explore alternative alliances but if their efforts come to nothing vote-weary Spaniards will be asked to return to the polls in December. 

The PP, in power since 2011, won the most seats in elections held in December and June but fell short of a majority both times as voters angry over corruption and austerity flocked to new parties.

Spain has never had a coalition government since it returned to democracy following the death of longtime dictator Francisco Franco in 1975 and the country's four main parties have been unable to cobble together a governing alliance.

In his speech before the second vote, Rajoy made a final appeal to Socialist MPs to support his bid to form a government, and so avoid 'running the risk of having to return to the polls'.

"We all have the obligation to ensure that Spain does not foot the bill for parliamentary rifts and stubbornness," he added.