Concerns that individuals named could be targeted by the Taliban led the previous Conservative government to set up the relocation scheme, involving thousands of people and estimated to cost the government 2 billion pounds ($A4.1 billion).
UK defence minister John Healey apologised for the data leak, which included details about members of parliament and senior military officers who supported applications to help soldiers who worked with the UK military and their families relocate from Afghanistan.
"This serious data incident should never have happened," Healey told MPs in the House of Commons.
"It may have occurred three years ago under the previous government but to all whose data was compromised I offer a sincere apology."
The breach resulted in the creation of a secret Afghan relocation scheme - the Afghanistan Response Route - by the government in April 2024.
Healey said that about 4500 affected people "are in Britain or in transit ... at a cost of around 400 million pounds".
But he added that no-one else from Afghanistan will be offered asylum because of the data leak.
News of the leak comes as the UK's public finances are tight and the populist Reform UK political party leads in the opinion polls.
UK forces were first deployed to Afghanistan in 2001 following the September 11 attacks on the United States and they played a major role in combat operations there until 2014.
The government is facing lawsuits from those affected by the breach, further adding to the ultimate cost of the incident.
A Ministry of Defence-commissioned review of the data breach, a summary of which was also published on Tuesday, said more than 16,000 people affected by it had been relocated to the UK as of May this year.
The details emerged on Tuesday after a legal ruling known as a superinjunction was lifted.
An unprecedented superinjunction was made at the High Court in September 2023 after the MoD argued that a public disclosure of the breach could put people at risk of extra-judicial killing or serious violence by the Taliban, with the decision to apply for an order made by then-defence secretary Ben Wallace.
The superinjunction, lifted on Tuesday, is thought to be the longest lasting order of its kind and the first time the government has sought such a restrictive measure against the media.
At multiple hearings, lawyers for the MoD said in written submissions that there was a "very real risk that people who would otherwise live will die" if the Taliban gained access to the data.
Healey, the Defence Secretary, told MPs the superinjunction had left him feeling "deeply uncomfortable to be constrained from reporting to this House" about the breach and the secret relocation scheme set up in its wake.
Under plans set out last October, the Afghanistan Response Route was expected to allow up to 25,000 people - most of whom were ineligible for Arap but deemed to be at the highest risk from Taliban reprisals - to be relocated.
Judge Martin Chamberlain, who ruled that the injunction should be lifted, said on Tuesday at the High Court that the gag order had "given rise to serious free speech concerns".
"The superinjunction had the effect of completely shutting down the ordinary mechanisms of accountability, which operate in a democracy," he said.
"This led to what I describe as a 'scrutiny vacuum'."
The dataset contained personal information of nearly 19,000 Afghans who had applied to be relocated to the UK and their families.
A High Court said in a summary of its ruling to lift the injunction that the data "contained personal information about more than 33,000 applicants".
It was released in error in early 2022 before the MoD spotted the breach in August 2023 when part of the dataset was published on Facebook.
The former Conservative government obtained the injunction the following month.
Prime Minister Keir Starmer's Labour government, which was elected last July, launched a review into the injunction, the breach and the relocation scheme, which concluded that although Afghanistan remains dangerous, there was little evidence of intent by the Taliban to conduct a campaign of retribution.
with PA and AP
