A major disaster was narrowly averted at Sellafield's nuclear waste plant in Cumbria when more than 2,000 tonnes of high-level nuclear waste almost exploded, The Observer can reveal.

Safety procedures were so lax at one of the plant's waste storage facilities last month that workers ignored alarm warnings of a build-up of explosive gases for nearly three hours. The gases were accumulating in the 21 tanks that each store 100 tonnes of deadly waste.

Ten hours longer, experts say, and the tanks would have been explosive.

Ministers were briefed last Tuesday about the incident, the worst in a long line of safety scandals at the plant. Peter Hain, the Minister for Energy, was informed by the Government's nuclear safety team, the Nuclear Installations Inspectorate (NII), that an incident had occurred at the plant on 26 January.

The NII has launched a major investigation, sending four inspectors to the plant and requesting that the owner of the plant, British Nuclear Fuels Limited (BNFL), report on safety procedures within four weeks.

'This is an extremely alarming situation,' said Andrew Stunnel, the Liberal Democrat spokesperson on energy. 'It shows the cavalier attitude that has dogged the nuclear industry for 50 years.'

Martin Forewood, a campaign co-ordinator for the local activist group Core, who has lived in the area for 25 years, said:'We're constantly reassured by the Government and BNFL that we have nothing to worry about. This incident shows how close we can come to a catastrophic incident.'

BNFL admitted its staff ignored alarms for two-and-a-half hours, and said the situation was 'inadequate'. It said it would take a lot longer than 10 hours for the situation to become critical and risk an explosion. It confirmed that the reprocessing plant at Thorp had been shut down as a result of the incident.

'Those 21 tanks of waste contain huge quantities of the most hazardous materials on the nuclear site, if not the planet' said John Large, one of the world's leading nuclear engineers. 'A similarly sized tank blew up in the Russian area of Chelyabinsk in 1957 and on its own devastated an area the size of central London.

'The only other time something like this has happened in Europe was in France in the Seventies when they rushed to use emergency military generators to cool the tanks and prevent an explosion.'

The incident happened at 8.30pm when engineers were carrying out improvements on the ventilation systems that stop the explosive gases from building up inside the tanks. The engineers incorrectly wired the circuits in the ventilators.

nick.walsh@observer.co.uk