The former Guardian journalist Donald MacLeod, who was left brain-damaged after he was hit by a police car that was answering a 999 call while he was cycling home, has won a high court claim that the Metropolitan police are liable for damages.

MacLeod was in a coma for weeks and needs 24-hour care following the accident in north London four years ago. His family's solicitor says they will oppose any possible appeal and will now prepare a claim for damages.

MacLeod, a keen and safety-conscious cyclist who used his bike daily, was wearing a helmet and a high-visibility jacket with his bicycle lights illuminated, said the judge Martin McKenna, who ruled that there could be no question of contributory negligence.

At the point where MacLeod was hit, at a mini-roundabout, the road narrowed and he would have broken the Highway Code if he had ridden on the pavement. The judge said it was the responsibility of the police driver, who was on his way to a shooting in Hackney with three other officers, to pass the cyclist safely. MacLeod could not be criticised for failing to take measures to avoid the impact, despite the car's siren and flashing lights.

The officer, who entered the roundabout at 55mph, was driving at an excessive speed in circumstances where he could not bring the car to a halt in time to avoid the accident, against the Met's own policy, the judge said.

"The manner of his driving plainly fell below an acceptable standard and he failed to drive with such care and skill as was reasonable in all the circumstances," McKenna said. "His speed was high and consistent with a desire to get to the rendezvous point as his priority rather than safely.

"The reality is that he would have arrived at the rendezvous point within the expected response time if he had driven to the speed limit. But for the breach of duty, the injury to the claimant would not have occurred."

MacLeod, 63, who has three children, was an education journalist at the Guardian before becoming head of communications at the Russell group of universities. He was cycling home after having two small glasses of wine with a friend at El Vino wine bar in Fleet Street in March 2010.

The family have since moved to Inveresk, near Edinburgh. His wife, Barbara, who brought the case on MacLeod's behalf, said: "It has been such a rollercoaster. We couldn't believe the police would force us to take them to court. It is such a relief to know that Don's care is now secure and we don't have to worry about that in the long term. It has been such a difficult time."

Although her husband "is as good as he has been" since the accident and can communicate through smiles, nods and shakes of the head, she said he was unable to speak and had to be moved in a wheelchair.

"I threw my arms round him when we got the message [from the court]. I don't know if he fully grasped how important the announcement is. He is physically well, getting some good physiotherapy and getting stronger all the time. Hopefully a change in his drug regime will help with communication."

MacLeod's medication had to be altered about a year ago because of seizures. At that time, his wife said: "It was not just odd words. He was responding with new vocabulary to what we had said to him. We desperately hope we can get back to that."

Meanwhile, she said, "we are getting out a lot, he enjoys being out and I am hoping to get an adapted vehicle so we don't have to use a taxi all the time."

She said they hoped to get a wheelchair with better suspension "so we can be a bit more adventurous". They would soon be going to the ballet with friends. "We are beginning to do normal, everyday things."