David Cameron has said there should not be a "knee-jerk reaction" to changing the laws on gun ownership after 12 people were shot dead in Cumbria.
The prime minister said everything must be done to make sure it "cannot happen again", but existing controls were among the "toughest" in the world.
He was speaking after Home Secretary Theresa May said the government would "consider all the options" on gun laws.
But she told MPs it would be "wrong to react before we know the full facts".
Mr Cameron, who is to visit Cumbria on Friday, told journalists at a joint press conference with Canada's PM Stephen Harper "today is a day for mourning and prayer".
He said his thoughts were with all those affected by the "absolutely horrific events" which had left a "close-knit" group of communities "deeply torn".
'Flick a switch'
The government would do everything it could to help "mend the hurt" caused by the incident, he said but while it was right that laws and practices on gun ownership should be looked at, he said there was not always "an instant legislative or regulatory answer".
He stressed that gun licences had to be regularly reviewed and the procedures for doing so were strict.
"Of course we should look at this issue but we should not leap to knee-jerk conclusions on what should be done on the regulatory front. We do have some of the toughest legislation in the world," he said.
He added: "You can't legislate to stop a switch flicking in someone's head and this sort of dreadful action taking place."
It is not yet known what prompted Derrick Bird to drive around shooting people - leaving 11 injured as well as the 12 dead.
Mrs May told MPs it was "right and proper" to have a debate after the killings but warned it would be "wrong to react before we know the full facts".
She said more than 100 detectives were taking part in the investigation and would be looking into Bird's history, his access to firearms and the "motivation for his actions". Police have confirmed that he had licences for the two weapons he used - a shotgun and a .22 rifle.
A number of government departments were standing ready to provide support to local police, the local authority and charities, she said.
She said MPs' thoughts would go out to the "family and friends of those so senselessly killed and injured" in the shootings and paid tribute to the work of the emergency services.
"Undoubtedly, yesterday's killings will prompt a debate about our country's gun laws, that is understandable and indeed it is right and proper," she said.
"But it would be wrong to react before we know the full facts. Today we must remember the innocent people who were taken from us as they went about their lives, then, we must allow the police time to complete their investigations."
She said once the police the government would "enter into, and lead, that debate" adding: "We will consider all the options." MPs would get the chance to take part before the summer recess, she said.
Mrs May added that mass killings were "extremely rare" in Britain but said: "That doesn't make it any the less painful, and it doesn't mean we shouldn't do everything we can to stop it happening again."
"Where there are lessons to be learned, we will learn them. Where there are changes to be made, we will make them. "
Shadow home secretary Alan Johnson said these were "dark times for a strong a close-knit community" and sent his condolences to those affected by the "tragic" events.
He agreed there should be no rush to change firearms laws but said they should be reviewed.
He said there should be a focus on whether follow-up checks on licence holders were "adequate" and whether there should be a greater role for GPs and the NHS in assessing whether gun owners' mental health was deteriorating.
Lib Dem MP John Pugh questioned how a "simple taxi driver could possibly justify the apparently lawful possession of such a formidable and devastating arsenal" and how killings could be prevented in future. Mrs May said it would not be right for her to comment before the police inquiry was completed.
Labour's John Woodcock - the MP for neighbouring Barrow and Furness - asked for any review to look at, not only firearms legislation but at local mental health services and to look at "how an apparently reserved member of the community suddenly snapped and was capable of such evil deeds".
And Labour MP Chris Williamson said the government should not "rule out the possibility of the complete prohibition of private ownership of firearms as the best way of preventing future atrocities like this".
However Labour MP Kate Hoey Britain already had "the most stringent gun control laws in Europe". And Conservative MP John Whittingdale said the "vast majority" of British gun owners shared the "dismay" about the events in Cumbria.