Leaked letters from emergency planners indicate a no-deal Brexit may mean Londoners face disruption to food and fuel supplies.
The London Resilience Forum letters highlight various concerns if the UK leaves the EU without a deal.
There could also be a shortage of chemicals needed to treat sewage, the LRF said.
Water companies said “necessary actions” were being taken to minimise disruption to customers.
The LRF said “vital” chemicals cannot be stockpiled because of their shelf life, the letters seen by the BBC say.
One LRF letter sent to the government said: “The Environment Agency and water utilities have raised with us the potential for disruption to public water and sewerage utilities in the event that border disruption or regulatory issues cause disruption to the import of chemicals and other resources required for water and wastewater treatment.
“We understand that chemicals vital to the continuity of these services are imported and restrictions including limits on amounts of chemicals permitted to be stored at sites in the UK and the shelf life of these products may prevent stockpiling being a solution to the problem.”
The letters said the water problem could be a national issue.
Water UK, responding on behalf of Thames Water and other local water companies, said: “Similar to other industries, we have been assessing the potential impact of Brexit – including a no-deal scenario – and we are taking the necessary actions to minimise disruption to customers and our water supply.”
Other issues highlighted in the letters included:
- Border hold-ups could lead to “a shortage of fresh meat, fruit and vegetables”
- There could be three months of border disruption at airports and ports
- London’s fuel supply could be affected “more than any other region” if border disruption causes “significant” travel disruption at the Dartford crossing in Kent
The LRF said it does not comment on leaked documents.