(Reuters) - Greek Prime Minister Alexis Tsipras will hand in his resignation later on Thursday and propose early elections be held on Sept. 20, government officials said. Tsipras was elected only in January but has suffered a rebellion among hard left lawmakers in his Syriza party and had to rely on opposition support to win parliamentary approval for Greece's new 86 billion euro bailout deal. - According to the constitution, if a government resigns within a year of the last election, the country's president must hand an exploratory mandate to the leaders of the opposition to try to form a government, starting with the largest party.