A 51,000-tonne ship which was deliberately grounded on a sandbank in a busy shipping lane to stop it from capsizing was loaded in a way that made it unstable, an accident report says.

Hoegh Osaka was carrying more than 1,400 cars and 105 pieces of construction equipment when it started listing off the Hampshire coast on January 3 last year shortly after it left Southampton port.

The crew beached the vessel deliberately on Bramble Bank sandbank, in the middle of a busy shipping lane, to save it from sinking.

An investigation by the Marine Accident Investigation Branch (MAIB) into the incident found that the crew failed to detect the vessel's instability before setting sail.

The upper decks of the vessel were full while the lower decks were only "lightly loaded", the report said.

Investigators also found that the estimated weight of many items of cargo was less than their actual weight.

After 19 days stranded, Hoegh Osaka was towed back to Southampton. Credit: PA

The report stated: "No allowance was made for the VCG (vertical centre of gravity) of the cargo loaded being above deck level."

"Hoegh Osaka's inadequate stability had not been identified prior to departure," the MAIB concluded.

Ensuring that a ship does have adequate stability was described as "a fundamental principle of seamanship that must not be neglected".

At the time, the crew were praised for their "great skill" by the chief executive of Hoegh Autoliners for deliberately grounding the vessel, leaving most of the cargo unscathed.

It was stuck in the Solent for 19 days before being towed back to a dock in Southampton.