Indoor gymnasiums, swimming pools and sports facilities in England have started to reopen in the latest easing of the coronavirus lockdown.
But the leisure industry has warned it will still struggle due to the financial impact of the pandemic.
At least a third of public facilities are expected to remain closed.
Gyms must follow strict hygiene and social distancing measures, such as limiting the number of people using the facility and the spacing of equipment.
Facilities should also reduce class sizes and provide adequate ventilation, according to government guidelines released earlier this month.
Swim England has issued separate guidance for operators on how to reopen indoor swimming pools, including implementing a one-way entry and exit system.
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Indoor gyms in Northern Ireland opened earlier this month, but they remain closed in Scotland and Wales.
Meanwhile, Health Secretary Matt Hancock has confirmed gyms and sports facilities will not reopen in Luton or Blackburn with Darwen due to an increase in coronavirus cases in those areas.
Gyms and outdoor swimming pools have been open in England since July 4 because there is less risk of catching coronavirus in the fresh air.
By Katy Austin, BBC News Business Correspondent
Will people return to gyms, swimming pools and recreation centers? After spending time and money preparing their facilities, this is the question that preoccupies those who run the venues.
Confidence is crucial – how secure people feel and how much they are willing to spend in a time of economic uncertainty.
Like other industries, fitness companies have suffered a financial blow. A report by trade body UKActive said membership cancellation rates were 15-23% due to the impact of the pandemic.
Some fitness professionals have adapted by introducing online classes and personal training, which have proven to be popular.
But UKActive says that although today is a milestone, the sector is “not out of the woods yet”.
He says support will still be needed with costs such as retroactive rent in the coming months, and that some facilities and services will remain closed due to continued financial pressures and restrictions.
Despite the relaxed rules, industry bodies have warned that many facilities could be forced to shut down amid the pandemic.
Community Leisure UK, the membership association which specializes in representing leisure and cultural charitable trusts across England, Scotland and Wales, estimates that 48% of all public leisure facilities are at risk to be closed, which means that as many as 1,300 could be lost by the end of the year, with more than 58,000 jobs.
The organization said about a third could not reopen on Saturday due to increased costs resulting from lost revenue during the lockdown and reduced capacity operation.
“The main reason is financial sustainability, because like all high streets and services – no income since the end of March,” said Mark Tweedie, Managing Director of Community Leisure UK.
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The District Council Network (DCN) also warns of the “uncertain future” facing gyms and recreation centers, noting that the sector is expected to lose around 305 million pounds this year.
The DCN, which represents 187 district councils in England that provide leisure services, is calling on the Treasury to provide a rescue program to save the leisure centers from collapse.
Indoor recreation centers and gymnasiums, as well as swimming pools and other indoor sports facilities have been closed since March 21 as part of measures to prevent the spread of the coronavirus.