The previous government set up a secret Afghan relocation scheme after the personal data of thousands of people was inadvertently leaked, it can be revealed.

The details of nearly 19,000 people who had applied to move to the UK after the Taliban takeover of the country were released by a British defence official in February 2022.

The Ministry of Defence (MoD) learned of the breach in August 2023 and created a new resettlement scheme nine months later. It has seen 4,500 Afghans arrive in the UK, with a further 600 people and their immediate families still to arrive.

The existence of the leak and scheme was kept secret for more than three years after the government obtained a superinjunction.

Details of the major data breach, the response and the number of Afghans granted the right to live in the UK as a result were only made public on Tuesday after a High Court judge ruled the gagging order should be lifted.

The leak contained the names, contact details and some family information of people potentially at risk of harm from the Taliban.

The government also revealed on Tuesday:

• The secret scheme – officially called the Afghan Relocation Route – has cost £400m so far, and is expected to cost a further £400m to £450m
• The scheme is being closed down, but relocation offers already made will be honoured
• The breach was committed mistakenly by an unnamed official at the MoD
• People whose details were leaked were only informed on Tuesday

Speaking in the House of Commons, Defence Secretary John Healey offered a “sincere apology” to those whose details had been included in the leak, which came to light when some details appeared on Facebook.

He said it was as a result of a spreadsheet being emailed “outside of authorised government systems”, which he described as a “serious departmental error” – though the Metropolitan Police has already decided a police investigation was not necessary.

Healey said the leak was “one of many data losses” related to the Afghanistan evacuation during that period, and contained the names of senior military officials, government officials, and MPs.

The MoD has declined to say how many people may have been arrested or killed as a result of the data breach, but Healey told MPs an independent review had found it was “highly unlikely” an individual would have been targeted solely because of it.

He said that review had also judged the secret scheme to be an “extremely significant intervention” given the “potentially limited” risk posed by the leak.

In a High Court judgement issued on Tuesday, Mr Justice Chamberlain said it was “quite possible” that some of those who saw the Facebook post containing the leaked personal data “were Taliban infiltrators or spoke about it to Taliban-aligned individuals”.

BBC News has seen an email sent to those impacted by the breach, which urges them to “exercise caution”, and take steps like protecting their online activities and not responding to messages from unknown contacts.

Healey said those who have been relocated to the UK have already been counted in immigration figures.

Tuesday’s disclosure dates back to the August 2021 withdrawal of US troops from Afghanistan, which saw the Taliban retake power and quickly surround the capital Kabul.

The leak involved the names of people who had applied for the Afghan Relocations and Assistance Policy (Arap) scheme, which the UK government set up to rapidly process applications by people who feared reprisals from the Taliban and move them to the UK.

The evacuation has already been heavily criticised in the years since it was launched, with a 2022 inquiry by the Foreign Affairs Committee finding it was a “disaster” and a “betrayal”.

When the government set up a new relocation scheme last year in response to the leak, members of the press quickly learned about the plans.

The government asked a judge to impose a superinjunction on the media, preventing outlets by law from reporting any detail.

Healey told the House even he had been prevented from speaking about the breach because of the “unprecedented” injunction, after being informed while still shadow defence secretary.

Reading a summary of his judgment in court, Mr Justice Chamberlain said the the gagging order had “given rise to serious free speech concerns”.

He continued: “The superinjunction had the effect of completely shutting down the ordinary mechanisms of accountability which operate in a democracy.

“This led to what I describe as a ‘scrutiny vacuum’.”

Shadow defence secretary James Cartlidge, who was in government when the secret scheme was established, said “this data leak should never have happened and was an unacceptable breach of all relevant data protocols”.

Erin Alcock, a lawyer for the firm Leigh Day, which has assisted hundreds of Arap applicants and family members, called the breach a “catastrophic failure”.