Britain would run out of clean drinking water within days of a no-deal Brexit in a doomsday scenario that convinced Michael Gove to back Theresa May’s deal.

Whitehall disaster planners have warned Ministers that leaving the EU without a deal could spark a national crisis as crucial chemicals used in water purification are imported to the UK from Europe.

The deliveries risk getting caught in weeks of border chaos if Britain quits the EU next March without the Prime Minister’s deal with Brussels being approved by MPs.

The vital chemicals are timed to arrive ‘just in time’ and cannot be stockpiled as they are too volatile, meaning water plants would have to turn off the taps as soon as they ran out or risk poisoning millions.

Offices and schools would close and hospitals plunged into chaos.

The startling warning is contained in secret Whitehall contingency plans codenamed Operation Yellowhammer leaked to this newspaper.

Michael Gove, the Secretary of State for Education, was said to have back Mrs May's Brexit deal after being warned Britain could run out of clean drinking water

The scale of the devastation outlined by the Government’s Civil Contingencies Secretariat is understood to be a key factor in convincing Cabinet Brexiteers led by the Environment Secretary that a deal must be sought at all costs.

Mr Gove has subsequently warned more junior ministerial colleagues that he has taken ‘the no deal pill’ and seen how bad things will get and judged that it is not a viable exit route.

And friends of the Vote Leave champion said that, as one of the architects of Brexit, he does not want the legacy of the referendum campaign to be death and chaos.

The threat of water shortages in hospitals was also behind Health Secretary Matt Hancock telling the Cabinet that he ‘could not guarantee that people would not die’ in the event of no deal.

Ministers were warned by 'disaster planners' that leaving the EU without a deal could spark a water crisis as crucial chemicals used in purification are imported from Europe to the UK

A Whitehall source told The Mail on Sunday: ‘This is Project Fear on steroids but it has worked on the Cabinet.’

In order to make water safe to drink, suppliers add chemicals, including fluorosilicic acid, aluminium sulphate, calcium hydroxide and sodium silicofluoride.

The Operation Yellowhammer contingency plans warn that if water plants run out of them they ‘would probably need to stop the water supply to all their customers – potentially millions of people’.

The doomsday vision would see householders ‘immediately face a shortage of drinking water and inability to flush toilets, cook, wash clothes or keep themselves clean’.

It adds: ‘Millions of people would not be able to go to work because office buildings, schools and other workplaces would need to close.’