Labour politicians are calling for answers from the government over the deadly fire that has ripped through a West London tower block.
Twelve people have died in the Grenfell Tower blaze, with the Met Police saying that figure was "likely to rise".
The prime minister said she was "deeply saddened by the tragic loss of life".
Jeremy Corbyn said it was "everyone's worst nightmare" and praised the emergency services but added that questions needed to be answered.
A review of building regulations covering fire safety was promised by Prime Minister Theresa May's chief of staff, Gavin Barwell, last year, when he was a government minister, but has not been published.
The Labour leader told the BBC: "At this stage, let's save life, let's bring safety to people at Grenfell Tower.
"I think tomorrow is a day for searching questions - the searching questions on the provision of fire safety equipment, the provision of sprinklers and the support the emergency services need and must have in all circumstances.
"You can never compromise on safety where fire is concerned."
The questions "may well be difficult" for the government, he added, saying: "The reports that were made in the past have to be acted on."
There have been reports that cladding - added in a recent refurbishment - may have helped the fire spread externally.
Checks are to be carried out on tower blocks going through similar refurbishment to Grenfell Tower, policing and fire minister Nick Hurd has said.
London Mayor Sadiq Khan said questions needed to be answered about the safety of tower blocks after some residents said they had been advised they should stay in their flats in the event of a fire.
A local residents' association had previously warned it was worried about the risk of a serious fire in the block.
"What we can't have is a situation where people's safety is put at risk because of bad advice being given or if it is the case, as has been alleged, of tower blocks not being properly serviced or maintained," said the mayor.
The government is not able to make a Commons statement on the tragedy as Parliament is not sitting following the general election.
But Commons Speaker John Bercow has said MPs will be allowed to question a minister on the subject on Thursday at a special meeting in the Palace of Westminster.
Labour MP Harriet Harman said she feared lessons had not been learned from a similar fire six years ago at the Lakanal House tower block in her Camberwell and Peckham constituency, which claimed six lives.
She told BBC Radio 4's the World At One programme: "Councils want to fit sprinklers in their tower blocks, but it comes down to money.
"The government has been cutting the money to councils. If you cut money to councils, you can't put in sprinklers."
Former chief fire officer Ronnie King, honorary secretary of the all-party parliamentary group on fire safety and rescue, said fire regulations "badly need updating" and "three successive ministers have not done it".
He suggested a government drive to cut red tape - by insisting that three regulations are removed for every new one created - should be reconsidered "where fire safety is concerned".
The all-party parliamentary group had recommended fitting sprinklers to buildings to save lives.
Judge Frances Kirkham, who presided over the inquest into the Lakanal House fire deaths, recommended the government should "encourage providers of housing in high-rise residential buildings" to "consider the retrofitting of sprinkler systems".
Kensington and Chelsea council, which owns the block, said it was focusing on supporting the rescue and relief operation but the causes of the fire would be fully investigated.
Cabinet minister David Gauke said: "I think it is far too early to jump to any conclusions.
"But of course we are going to need to look very closely at precisely what has happened to see whether there are measures to ensure that nothing like this should happen again.
"But today I think our focus has to be on providing support to those who have been injured."