While the diesel powered submarine made it back to Mumbai on its own after the incident, sources say that it suffered damage to its fins, periscope and aerials and will remain out of service for at least two months. The merchant ship, identified as MV Leeds, did not suffer any damage in the accident.

A court of inquiry has been ordered into the incident but preliminary findings indicate that the submarine suffered a collision as it was trying to surface in shallow waters during the Western Fleet war games. All 53 crew members on board are unhurt in the incident.

Sources say that the 2,500 tonne submarine had switched off sonar as it was in ‘restricted waters’ and its radar was not transmitting as part of the simulated exercise to escape any attention from passing vessels.

“The submarine performed a periscope scan and then proceeded to surface. It seems that the merchant ship did not get detected during the periscope scan. The submarine was operating at a precarious depth and could not dive to escape due to the restricted waters,” a Navy officer said.

However, the mysterious accident is bound to raise controversy as the collision took place in broad daylight — sources say that the ‘minor mishap’ happened on Monday afternoon. Initial findings also say that the merchant ship was not at fault and was operating as per the rules of the sea.

The damaged sub was escorted to Mumbai and made it back to the harbour only by Wednesday night. The Sindhughosh had recently undergone a massive two-year mid-life refit in Russia, costing over $ 80 million. It is equipped with Russian Klub anti-ship cruise missiles and is the only one in its class that is known to carry land attack cruise missiles. It is also fitted with an Indian sonar system.

In a similar case just over three years ago, another Indian Navy submarine suffered significant damage after colliding with an ‘unknown object’ while surfacing off the Goa coast. The subsequent court of inquiry could not establish the reason for the collision nor the object that had struck the submarine.

In the past few years, the navy has been hit by an increasing number of collisions and near misses involving its warships. Most of these events have occurred off the busy Western Coast that has a high density of shipping traffic.