The waste has been leaking into the ground from a storage silo at the Sellafield facility in Cumbria, in northwest England, since 2018 -- enough to fill an Olympic-sized swimming pool every three years.

Sellafield, which began operations in the 1940s, generated nuclear power between 1956 and 2003.

In a report published Wednesday, a group of British MPs criticised the pace of the decommissioning work, citing examples of "failure, cost overruns and continuing safety concerns".

It said the risks were highlighted by the Magnox Swarf Storage Silo (MSSS), described by Sellafield owner the Nuclear Decommissioning Authority (NDA) as "the most hazardous building in the UK".

The report by the Public Accounts Committee found that most of the annual targets for retrieving waste from buildings at the site, including the MSSS, had been "missed".

It said the storage silo is likely to continue leaking until the oldest section of the building has been emptied in the late 2050s, instead of 2040 as previously expected.

"Every day at Sellafield is a race against time to complete works before buildings reach the end of their life," said the committee's chair, Geoffrey Clifton-Brown.

"Our report contains too many signs that this is a race that Sellafield risks losing," he added, saying the site presented "intolerable risks".

The radioactive leak is contained and does not pose a risk to the public, the report noted.

A spokeswoman for the NDA said fixing it was the group's "highest priority".

Britain's National Audit Office forecast last October that the cost of decommissioning and cleaning up the entire Sellafield site over the next century had soared to pound136 billion ($177 billion).

This was up almost 19 percent on the previous official estimate made in 2019.

It noted that "full site remediation" was not expected until 2125.