ZURICH, Aug 20 (Reuters) - Swiss voters are set to approve a 6 billion Swiss franc ($6.6 billion) purchase of new fighter jets in a referendum next month, a poll for broadcaster SRF showed on Thursday.

The survey by gfs.bern found 58% of respondents favoured the government's plan while 39% opposed it and 3% had no opinion. The vote is planned for Sept. 27 and will decide whether to approve the purchase without knowing who will win the contract.

The poll of 29,450 eligible voters had a margin of error of 2.9 percentage points.

European aerospace group Airbus, France's Dassault , Sweden's Saab, and Boeing and Lockheed Martin from the United States bid for the contract in January. Swiss authorities later excluded Saab from the race, saying its entry was not developed enough.

Switzerland's ageing stable of Boeing McDonnell Douglas F/A-18C and D Hornets and Northrop F-5 Tigers is scheduled to be retired within years. Airbus's Eurofighter, Boeing's F/A-18 Super Hornet, Dassault's Rafale, and F-35As made by Lockheed Martin are in the running.

New jets are to be delivered by 2025.

Neutral Switzerland, which last fought a short war in 1847, has struggled in the past to persuade citizens to back a deal for new fighters. In 2014, around 52% voted against a 3.5 billion franc plan to buy 22 Gripen fighter jets from Saab.

($1 = 0.9094 Swiss francs) (Reporting by Michael Shields; Editing by Kirsten Donovan)