At the time, the Ministry of Defence called the incident a “suspected hijacking”, but prosecutors now say there was no organised attempt to take control of the vessel.
Seven men from Nigeria, where the ship had set sail weeks before, were detained and two were charged with conduct endangering ships, under the Merchant Shipping Act 1995.
Matthew John Okorie, 25 and Sunday Sylvester, 22, were held in custody ahead of a hearing due to take place at Southampton Crown Court on 29 January.
The five other stowaways had been arrested on suspicion of seizing or exercising control of a ship by use of threats or force.
But on Friday, Hampshire Police and the Crown Prosecution Service (CPS) announced that all seven men would face no further criminal action.
Senior district crown prosecutor Sophie Stevens said: “After additional maritime expert evidence came to light, we concluded there was no longer a realistic prospect of conviction and discontinued the case.”
The Independent understands that despite initial reports of an attempted hijacking, there was no organised attempt to take control of the ship.
The stowaways had been discovered by the Nave Andromeda’s crew days before the ship entered UK waters and had been given clothing, food and shelter.
On the day of the incident, The Independent was told that the men had become violent as crew members tried to detain them in a cabin.
But evidence uncovered during the investigation indicated that their actions did not amount to violent resistance, and expert analysis undermined accusations that the stowaways had endangered the ship or those on board.
Hampshire Police initially said it received reports that stowaways had made “verbal threats” to its crew.
“Initial reports had indicated there was a real and imminent threat, but additional mobile phone footage and further expert analysis of the evidence cast doubt on whether the ship or the crew were put in danger,” a spokesperson for the CPS said.
“As the evidence could not show that the ship or crew were threatened, the legal test for the offence of conduct endangering ships under section 58 of the Merchant Shipping Act 1995 was no longer met.
“On that basis we concluded there was no longer a realistic prospect of a conviction for this offence and discontinued the case.”
The SBS raid was authorised by defence secretary Ben Wallace and home secretary Priti Patel after a 10-hour incident when the ship remained off the Isle of Wight.
The 748ft (228m) Nave Andromeda had been heading towards Southampton, having set sail from Nigeria.
The ship's operator, Navios Tanker Management, said the stowaways “illegally boarded” the Liberian-flagged tanker in Lagos.
Experts said stowaways are a common occurrence in Nigeria and that it was unlikely that those on board the ship intended to come to the UK, or knew its destination when they boarded.
Oil tankers are equipped with numerous security features to safeguard against hijacking attempts and protect their cargo, including safe rooms for the crew.
A Home Office spokesperson said the seven men were being dealt with under immigration processes and that anyone found to have no right to remain in Britain would be deported.
“We are disappointed by this decision,” he added. “It is frustrating that there will be no prosecution in relation to this very serious incident and the British people will struggle to understand how this can be the case.”
The Home Office said it was working with the CPS “urgently to resolve the issues raised by this case”, but would not provide further details.
The CPS makes charging decisions independently of the police and government based on its “full code test”, which considers the evidence, prospect of conviction and public interest.
The case came amid allegations of unprecedented political interference in the criminal justice system.
Senior judges and lawyers have criticised attacks by the prime minister and home secretary on immigration lawyers after cases that went against the government.