Police will now have to check someone's medical history before issuing a gun licence in the wake of the Plymouth shootings.
Police will also have to review an applicant's social media accounts.
An announcement from the Home Office confirmed all firearms applications must be accompanied by a medical document signed by a registered, practising doctor from next month.
In particular, any information on mental health, neurological conditions and substance abuse will be reviewed as part of the process.
It will be the first time police will be legally required to follow the guidance to help improve standards and consistency across the UK's forces.
Police will also have to review an applicant's social media accounts, financial history and carry out domestic violence checks in cases where it's believed more evidence is needed before authorising a licence.
The news comes after 22-year-old Jake Davison killed five people in the Keyham area of Plymouth earlier this year before turning the gun on himself.
The Independent Office for Police Conduct (IOPC) is now investigating how Devon and Cornwall Police handled his original application for a shotgun certificate in July 2017.
It is also investigating the force's decision to return Mr Davison’s shotgun and shotgun certificate in July last year after he admitted assaulting two youths in a park in Plymouth.
National lead on firearms licensing Debbie Tedds said: "Policing take this matter incredibly seriously, and any advancement on the already extensive checks will help to ensure that only those who are safe to carry a firearms licence will receive one."
Home Secretary Priti Patel added: "This new guidance prioritises public safety above all else.
"We have taken considerable care to ensure it is comprehensive and enforceable, having worked closely with the medical, policing and shooting sectors."
The British Medical Association, which helped develop the guidance, said it makes clear doctors are responsible for providing medical evidence but the police force will make the final decision on issuing the licence.