Rebels within Greece’s ruling party, the left-wing Syriza movement led by the prime minister, Alexis Tsipras, have announced they are breaking away to form a separate party called Popular Unity. Angry at what they see as a betrayal of the party’s anti-austerity principles, the 25 MPs announced their intention to form a new party in a letter to parliament the day after Tsipras resigned to pave the way for snap elections next month. Led by the former energy minister, Panagiotis Lafazanis, the new movement will be the third-largest group in the Greek parliament and could conceivably receive a mandate to try to form a new government.