Kweku Adoboli, the rogue trader who was sentenced to jail over Britain's biggest ever fraud, was released after he served less than half his jail term.

He was released last week from Maidstone prison in Kent, an immigration retention centre, but the Ghanaian citizen still faces ban from working in the financial services sector.

He spent almost nine months in jail before his conviction and seven-year jail sentence in November 2012. He was expected to be released before completing his full term, taking into account the good behaviour and time served in custody since his arrest in January 2012.

After his conviction, he spent time at the Verne prison on the Isle of Portland in Dorset before shifting to Maidstone.

Adoboli, a relatively junior trader at UBS, lost the Swiss bank £1.5bn (€2.1bn, $2.4bn) with illicit trades that he concealed from superiors. He created and used an undeclared fund of profits, termed the Umbrella, which had the effect of manipulating the desk's reported profit and loss.

Adoboli, the son of a UN diplomat, was found guilty of two counts of fraud, grave enough to be sentenced for 20 years in prison.

The rogue trades were revealed to the public after Adoboli sent his bosses an email detailing his activities in mid September 2011.

Nevertheless, he denied charges in the court and testified that managers had pressured him to take unauthorised risks in a relentless effort to generate profits for the bank.

He wept frequently during the trial, and the judge said his intention was not personal gain.