(Reuters) - Bulgaria has extended the deadline for submitting bids to run Sofia’s airport for a third time, to Feb. 5 following changes in the documents for the concession contract, transport ministry data showed on Wednesday.

The ministry has already pushed the timeline back several times from the original Oct. 22 due to numerous queries about the process and has now carried out some technical changes, a transport ministry spokeswoman said without elaborating.

The centre-right government re-launched the tender in July and is seeking to make 550 million euros ($630 million) from the concession over 35 years, 280 million of which should come in an upfront payment.

It expects the airport, the main air hub for the Balkan country which is now run by the state, to generate revenue of 3.46 billion euros over the whole period and up to 3.9 billion if extended to a maximum 46 years and seven months.

So far, Manchester Airports Group (MAG), Britain’s largest airport operator, has said it plans to bid. A spokesman for India’s GMR Group has said the group would consider bidding based on the final terms of the tender.

Germany’s Fraport has said it was interested in the process and Spain’s Aena has said it was studying a joint bid with Australian group First State.

French Aeroports de Paris, which has a 46 percent stake in Turkey’s TAV Airports as well as Switzerland’s Flughaven Zuerich have also been looking at it. ($1 = 0.8731 euros) (Reporting by Tsvetelia Tsolova; editing by David Evans)