Hungary on Monday backed a planned Russian gas pipeline crossing south-east Europe in a move that risks angering Budapest’s western partners and could threaten the prospects of Nabucco, a rival pipeline supported by the US and the European Union.

The move was announced by Ferenc Gyurcsany, the Hungarian prime minister, during a visit to Budapest by Dmitry Medvedev, Russia’s first deputy prime minister and likely successor to the president, Vladimir Putin. The deal is expected to be signed in Moscow on Thursday.