Nabucco, the ambitious pipeline proposal to bring natural gas to the European Union from the Caspian region, will pass its first ­significant milestone on Monday when an inter­governmental agreement between the project’s backers is signed.

Turkey, where the pipeline is intended to start, had been obstructing a deal but a solution has been found to meet the country’s demands for security of supply without making a legal commitment to provide it with a fixed amount of gas.