ATHENS (REUTERS) - Greek Prime Minister Alexis Tsipras' leftist Syriza will be the biggest party after elections on Sept 20, but most voters think he was wrong to seek a fresh mandate, according to the first major opinion poll published since he resigned last week.

Syriza was supported by 23 per cent of those polled, with the conservative New Democracy party second on 19.5 per cent, according to the survey, carried out by pollsters ProRata and published in Friday's Efimerida Ton Syntakton newspaper.

The previous ProRata poll in early July showed a wider gap in Syriza's favour, putting the party on 26 per cent compared with 15 per cent for New Democracy.

Popular Unity, the party formed last week by disaffected Syriza members who oppose the country's latest 86-billion-euro (S$135 billion) bailout, was backed by 3.5 per cent in Friday's poll - just above the 3 per cent threshold needed to enter parliament.

Independent Greeks, the ally in Tsipras' former coalition government, scored 2 per cent.

Based on the survey, 64 per cent of Greeks believe Tsipras' move to seek a fresh mandate through snap polls was wrong, while 68 per cent agree that Greece must stay in the euro zone at any cost, even if that meant further austerity.

"The answers to these two questions lead to the conclusion that early elections may cost the (former) prime minister and Syriza," the newspaper said.

One third of those who supported Tsipras' party in the January 2015 elections that took him into office said they were unsure if they will do so again, the paper said.

The poll also showed 25.5 per cent of voters were still undecided, making them the biggest bloc.