Two east London hospitals have asked patients to stay away after their emergency departments were hit by flooding on Sunday.
Whipps Cross is without power and evacuating 100 inpatients and Newham hospital is asking patients to use other A&Es for urgent care.
Ambulances are being redirected after torrential rain caused severe flooding in homes, roads and stations.
London Fire Brigade says it has taken more than 1,000 flooding-related calls.
It rescued people trapped in cars and is helping those with flooded basements and collapsed ceilings.
A spokesperson for Barts Health NHS Trust which runs both Newham and Whipps Cross hospitals, said: "Patients are asked to attend alternative hospitals where they can. A major incident has been called across the Trust."
Many of the capital's roads closed due to the flooding, including the Blackwall Tunnel, the A12 and parts of the North Circular.
Stepney Green station remains closed although eight other Tube and London Overground stations have reopened.
A yellow thunderstorm warning remains in place for parts of south-east England.