India is investigating the extent to which secrets about French Scorpene submarines being built in the country have been compromised, its defence ministry said on Wednesday, after a leak of documents relating to its combat capabilities.

The leak, which was first reported in The Australian newspaper, contains more than 22,000 pages outlining the secret capabilities of six submarines that French builder DCNS has designed for the Indian Navy.Excerpts published in redacted form on the newspaper’s website contained highly sensitive details of the submarine including technical manuals and models of the boat’s antennae.

The newspaper The Australian reported that the leaked data includes descriptions of what frequencies the submarines use to gather intelligence, where a crew can speak safely to avoid detection, and specifications of the torpedo launch system.

“I understand there has been a case of hacking,” Indian Defence Minister Manohar Parrikar told reporters.

“We will find out what has happened.”

Shipmaker DCNS said in a statement that French national security authorities are investigating the size, seriousness and cause of the leak.

Thales, which owns 35 per cent of the shipbuilder, could not immediately be reached for comment outside normal European business hours. The French defence ministry declined to make an immediate comment.

The Indian Defence Ministry said it was investigating the impact of the leak on the submarine programme which it said had occurred from abroad. It gave no details.

“The available information is being examined at Integrated Headquarters, Ministry of Defence (Navy) and an analysis is being carried out by the concerned specialists,” it said in a statement.

“It appears that the source of leak is from overseas and not in India.” Uday Bhaskar, a former naval officer, said that if the leak was established, it would amount to a significant compromise of the credibility of the submarines.