Social distancing will be impossible in some places of worship if government ministers allow them to reopen, religious leaders have warned.
The Anglican Bishop of London, the Rt Rev Sarah Mullally, said churches might not return to normal services before the end of the year.
Mosques, churches and temples in the UK have been closed for almost two months.
The prime minister is due to make a statement about the lockdown restrictions later.
It is not clear if the government will change its guidance for places of worship.
But senior religious leaders have told the BBC that faith communities will have to endure long-term changes to their worship in order to prevent the spread of coronavirus.
Ibrahim Mogra, a senior imam in Leicester, warned the prime minister not to ease restrictions on places of worship before the end of Ramadan, the Islamic holy month.
"I am not convinced that we can maintain social distance," he said.
"Within a mosque set up the first thing is the removal of footwear. And then it's the ritual washing, and then going into the main prayer hall," he said.
"We are talking about a five times regular daily attendance compared to other places of worship," he said. "So we are talking about really large numbers of people."
The Muslim Council of Britain, the UK's largest Muslim umbrella organisation, is consulting its members before issuing guidance this weekend for mosques that are considering reopening.
"The majority of the mosques that we have consulted are of the view that they do not wish to open during Ramadan," an MCB spokesman said.
"We do not want to be the ones who cause harm to others."
The Rt Rev Sarah Mullally, the Bishop of London, is leading the Church of England's planning for reopening its buildings. She said there would have to be significant changes to key aspects of Christian worship "for some time" to come.
"I don't envisage, even up to the end of the year, we will be back to our normal services.
"We'll have some churches doing things differently. And of course, this approach will depend on the part of the country you are in. Being in Devon is very different to being in the centre of London. So we need to approach this based on our local circumstances," she said.
"There are some very challenging questions that we'll have to face, not least about singing and about the receiving of Holy Communion. So the future will look different.
"But we want to continue to support people in their spiritual journey with their faith,"