In a long-awaited report, a panel of independent experts convened by the British government said both those theories were unlikely, and that it had been "unable to identify a clear and convincing single cause" for the deaths.

"We've ruled things out, but we've not been able to confidently rule things in...," panel member Tammy Horton, a researcher at the National Oceanography Centre, told an online press briefing. "We've come up with probably more questions than answers."

The report said the deaths occurred over a sustained period and along dozens of miles of coastline and involved unusual twitching of many crabs as they died, and that other marine life remained largely unaffected.

It was "about as likely as not" that a pathogen new to UK waters caused the deaths, it said, though no new pathogen was detected.

Several of the factors it considered might in combination also have been a cause, it added.

British Environment Minister Therese Coffey said on Friday she would consider if further analysis by government scientists might establish a conclusive cause.

"The focus now needs to be on understanding more about the novel pathogen identified as the most likely cause, and what support we can offer our local fishing industry as it recovers," Simon Clarke, a former cabinet minister and the local lawmaker in the region, said.