Japan Airlines has conducted unauthorized aircraft maintenance since 2001 and could be penalized, the Japanese government said on Wednesday.

JAL acknowledged it had repaired parts such as landing gear on 10 aircraft owned by group firm Japan Transocean Air and fixed parts on seven aircraft owned by Taiwan's Eva Airways without gaining necessary government approval.

Japanese law requires airlines to obtain government approval before carrying out maintenance on aircraft owned by other firms.

JAL also had orders to repair brake parts on six aircraft owned by J-Air, another group firm, but halted some remaining processes after finding this month that it had failed to gain the government's consent, JAL spokesman Tatsuo Yoshimura said.

An official at Japan's Trade Ministry said it was checking whether JAL's unauthorized maintenance violated the law.

If an unlawful act is confirmed, the ministry will first order JAL to follow the safety procedures, then the government may move onto penalties such as a fine, he said.

JAL had vowed to ensure it carried out safety procedures in response to an order by transportation authorities to improve operations due to recent lapses, including panels falling off in flight and a case of a plane taking off without permission.