The NHS has been declared 'open for business' but some hospitals are still suffering disruption caused by the crippling cyber attack.
Around 60 health service organisations in England and Scotland were paralysed by the cyber attack on Friday, which hit 200,000 victims in 150 countries around the world.
GP Out of Hours Service in Doncaster experienced telephone issues and Rotherham Council blocked all external emails.
Following a meeting of the Government's Cobra contingencies committee, Home Secretary Amber Rudd said more than a million patients had been treated in the course of Monday.
She added: "All GPs surgeries did open, though some of them had to use pen and paper. The vast majority of patients have noticed no difference. It has been a very strong response."
Earlier, Health Secretary Jeremy Hunt confirmed there had not been a second wave of attacks on NHS trusts and said it was "encouraging" that the level of criminal activity was at "the lower end of the range" anticipated.
Ms Rudd said the National Crime Agency and the National Cyber Security Centre were now part of an "international manhunt" to find the perpetrators.
She added: "NCSC and NCA are working with Europol and other international partners to make sure we all collect the right evidence, which we need to do to make sure we have the right material to find out who has done this and we go after them. Which we will." she said.