A controversial merger involving East Midlands Ambulance Service (EMAS) will result in improved services, health bosses have promised.
Four million people live in the area covered by the new service
EMAS already covers Notts, Leics and Derbys but the new service, which officially starts on Saturday, will also take in Northants and Lincs.
EMAS won one star out of three in the most recent performance tables.
Response time targets are not being met but bosses insist the merger and new investments will improve the situation.
The move means EMAS will be the biggest ambulance service outside London covering regions that are home to four million people.
David Whiting, director of operations at EMAS, said a bigger service would mean better, more targeted care for patients.
"Our ambition is to treat as many patients in the home and the community as we take to hospital so a lot of patients, particularly those with long-term conditions, would benefit from having their care provided very locally or in their own homes," he said.
Plans to merge the ambulance services were not well received with 80% of people who took part in a public consultation opposing the proposals.
The chairman of the Lincolnshire's ambulance trust board has said the move was "a waste of time".
Linda Honey told BBC News in May: "I am a loss to see the point of the consultation process as the majority of the respondents from across the East Midlands were against the proposals."
But the Department of Health said the merger was in patients' best interests and approved the plans.
The move forms part of the government's plan to cut ambulance services nationally from 29 to 11 in a bid to save an estimated £250m a year.