Sellafield, Europe’s most hazardous nuclear site, has a worsening leak from a huge silo of radioactive waste that could pose a risk to the public, the Guardian can reveal.

Concerns over safety at the crumbling building, as well as cracks in a reservoir of toxic sludge known as B30, have caused diplomatic tensions with countries including the US, Norway and Ireland, which fear Sellafield has failed to get a grip of the problems.