Faced with an ageing fleet, Bulgaria's parliament on Friday voted in favour of a controversial law amendment granting NATO jets the authority to help protect its airspace.

The vote passed by 96 to 34 after a heated debate among lawmakers.

Bulgaria's opposition, made up of socialists and ultra-nationalists, had been highly critical of the proposal, calling it "a violation of national sovereignty".

"The protection of the airspace will either be ensured by the Bulgarian army on its own or in collaboration with the armed forces of allied countries," the amended defence law now reads.

The move came just months after neighbouring Romania also gave NATO the right to safeguard its airspace.

As a NATO member, Bulgaria has an obligation to keep at least one squadron of 12 planes in good fighting order.

But currently only four of its 19 Soviet-built MiG-29 fighter planes are still in good working condition.

Despite strong opposition from Moscow, Sofia signed a deal with Warsaw last year to repair six of the jets in Poland, with the aim to reduce dependency on Russia.