Sunak has made "stopping the boats" one of his top five priorities and is seeking to revive a plan to send asylum seekers who arrive in Britain on small boats to Rwanda after it was blocked by the courts.
The government said earlier this month it had met a pledge made by Sunak to clear by the end of 2023 a so-called legacy backlog of 92,000 asylum claims which were made before a June 2022 change in immigration law.
But the opposition Labour Party disputed the government's statement, citing official statistics showing 4,537 of those asylum applicants were still awaiting an initial decision over their application as of Dec. 28.
The government said these were complex cases that required extra work, but Robert Chote, chair of the UK Statistics Authority, said the government had never previously argued that such cases should be excluded when calculating the backlog.
"It is not surprising that the government's claim has been greeted with scepticism and that some people may feel misled when these 'hard cases' remain in the official estimates of the legacy backlog," Chote said in a letter to opposition lawmakers.
"This episode may affect public trust when the government sets targets and announces whether they have been met."
Chote also criticised the government for not supplying journalists with the data behind the statement when it first said it had eliminated the backlog.
"This does not support our expectations around intelligent transparency, and we have raised this with the Home Office," he added.
Britain's interior ministry did not immediately respond to a request for comment.
(Reporting by Alistair Smout, additional reporting by Muvija M; Editing by Sharon Singleton)
