Numerous London schools have closed in icy conditions over the past week
London's mayor has urged schools to remain open as low temperatures prevented an ice thaw and continued to disrupt travel.
Boris Johnson said closing schools had a "devastating" impact on parents who cannot arrange last-minute childcare.
South West Trains and Chiltern rail services are disrupted and Eurostar is running a reduced service.
Significant delays have been reported at Heathrow and Gatwick airports but Tube services are running as normal.
London's Mayor referred to school closures due to bad weather in a speech he gave at Mansion House in the City of London on Thursday night.
Mr Johnson said: "I do urge head teachers and governors and anybody present here representing educational establishments wherever possible to show that London is not only open for business but for education as well."
Later the mayor added: "There are times when it is obviously not safe to carry on as usual but I do urge head teachers, governors and education authorities to consider the consequences of school closures on parents who are then prevented from going to work.
"This has a disproportionate impact on women and can be devastating for single parents when no alternative childcare arrangements can be put in place at short notice."
It came after numerous London schools were closed on Thursday, among hundreds of school closures across the country.
Many London schools re-opened on Friday but about 30 were shut in Harrow, 43 were closed or part-closed in Waltham Forest and at least seven closed in Croydon.
Parents were advised to contact schools or local education authorities to check.
The mayor's economic advisor, Anthony Brown, told BBC News: "Most people have been able to get into work and we urge the schools to think really hard before actually closing down because it has enormous knock-on consequences."
He added: "One school closing down because a couple of teachers can't make it in might mean that hundreds of parents don't get into work.
"That has a devastating impact on those people, particularly if they are freelance and they can't earn the money, but also on the wider London economy."
National Association of Headteachers general secretary Mick Brookes said: "Let's just remind Boris and others that schools are places of learning and not creches for children, that's the important factor.
"But of course we recognise that when the school is closed or other circumstances pertain, that childcare for working parents and particularly single working parents, is difficult."
Swimmers braved the frozen Serpentine lake in Hyde Park
Mr Brookes described school closures as a "pretty rare" occurrence.
"What happens more often of course is the illness of a child," he added.
"Parents have to make arrangements for when their child is ill to be looked after and I think this comes in the same category."
While the UK experienced its coldest night of the winter so far, with temperatures falling as low as -21.2C (-6.2F) in the north-west Highlands, in London they fell to -3C (27F).
South West Trains services have been cancelled between Waterloo and Dorking and between Virginia Water and Weybridge.
Southeastern Trains are running replacement bus services between Grove Park and Bromley North.
An underground cable fault in East Finchley, north London, left 968 homes without power for three hours until 0110 GMT.
EDF Energy said power has been restored and engineers are working to prevent the problem recurring.
Lewisham Hospital NHS Trust cancelled a number of operations on Friday because staff were unable to get to work.
A spokesman said: "Due to the continuing severe weather, snow and icy conditions, we have had to cancel any appropriate elective surgery, and some outpatient clinics, except for urgent appointments."
Despite the freezing temperatures, members of Serpentine Swimming Club swam in a hole in the ice in Hyde Park's Serpentine lake in central London.
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