UPDATE 1-Bulgaria to have early election in September or October
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* Sofia government likely to quit within weeks
* Centre-right opposition GERB party set to win polls
* Bulgaria at centre of East-West tussle over gas
SOFIA, June 17 (Reuters) - Bulgaria's political parties have agreed to hold an early election sometime between Sept. 28 and Oct. 12, President Rosen Plevneliev said on Tuesday.
The coalition of Prime Minister Plamen Oresharski is expected to resign within weeks, after the ruling Socialists did poorly in the European Parliament elections and a junior coalition partner withdrew support.
"The political parties agreed the need for a smooth transition to political stability and the need for further political consultations to set up early elections in the period between Sept. 28 and Oct. 12," Plevneliev said in a statement after consultations with parliamentary groups.
Analysts expect the centre-right opposition GERB party to win, though it may struggle to cobble together a stable coalition government.
The next government must walk a diplomatic tightrope over the fate of the Russian-led South Stream gas pipeline, whose proposed construction has thrust Bulgaria into the middle of a dispute between Moscow and the European Union.
The Socialists had backed the pipeline, angering Brussels which said it violated EU rules.
In a sign the next government could be cooler towards the project, GERB's leader said if he were to gain power he would scrap a contract with Russian firm Stroytransgaz - whose owner is on Western lists of sanctions targets - to build the Bulgarian leg of the pipeline. (Reporting by Matthias Williams and Tsvetelia Tsolova; Writing by Radu Marinas; Editing by Christian Lowe and Alison Williams)