Thousands of landing card slips recording the arrival of ‘Windrush generation’ migrants to the UK were destroyed eight years ago, the Home Office has confirmed.

The Labour Party condemned the “truly shocking” revelation first made by former Home Office employee to the Guardian.

The whistleblower said the records helped immigration officials when seeking information about when a person arrived in the UK, particularly in cases where a migrant was attempting to resolve problems with their immigration status.

The records were destroyed when Prime Minister Theresa May was in charge of the department as Home Secretary.

The Home Office claimed the landing card slips “did not provide any reliable evidence” on their immigration status.

In response, the Prime Minister’s spokesman said disposing of documents had been the right decision to take.

The spokesman said: “The slips provided details of an individual’s date of entry, but did not provide any reliable evidence relating to ongoing residence in the UK, or their immigration status and therefore would not have bearing on immigration cases whereby Commonwealth citizens are proving residency in the UK.”

He added: “It was done for, I think, reasons as well to protect personal information. The decision was taken to securely dispose of these documents, and that was the right one to take.”

Asked if May had been aware of the disposal while she was home secretary, the PM’s spokesman said: “My belief, at this moment, is that it was an operational decision that was taken by the Border Agency.”

Earlier on Tuesday, May said sorry to Caribbean leaders for the controversy.

A Home Office spokesman said: “Registration slips provided details of an individual’s date of entry, they did not provide any reliable evidence relating to ongoing residence in the UK or their immigration status.

“So it would be misleading and inaccurate to suggest that registration slips would therefore have a bearing on immigration cases whereby Commonwealth citizens are proving residency in the UK.”

The former employee said the landing card records, dating back to the 1950s, had been stored in the basement of a government tower block.

In October 2010, the Home Office decided to destroy the cards, when its base in Croydon was closed and the staff were moved to another, smaller site, he said.

Shadow Home Secretary Diane Abbott said the disclosure shows “the culpability of this fiasco rests solely with the Home Office”.

She said: “Amber Rudd must explain who authorised this action and when, and what Theresa May’s role was as the then Home Secretary.

“The Windrush generation have been threatened with deportation because they cannot provide documents, but now we learn that the Home Office destroyed the very records that could have demonstrated their right to remain.

“The Home Secretary should immediately announce that she will accept the burden of proof in deportation cases like these lies with the Home Office, not with the potential deportee.”