Non-urgent admissions were cancelled at Guy's and St Thomas's hospitals in London, and South Central Ambulance Service used four-wheel-drive vehicles to reach people in rural parts of Oxfordshire.
A spokeswoman reported that falls, breathing difficulties and road traffic accidents had slowed response times, while the London Ambulance Service was also under pressure and only dealing with life-threatening situations.
The number of calls hit 650 between midnight and 6am, which was significantly more than normal.
Great Ormond Street Hospital for Children in London said severe weather had prevented many staff getting into work, and urged people with outpatient appointments to stay at home.
The same advice was given to visitors and anyone due to attend the hospital for non-urgent operations.
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