A fleet of 19 new ADL Enviro400 Citys are destined for the fleet of Go-Ahead-owned Bluestar.
The Southampton operator will use the double-deckers on its popular route 18 – between Millbrook and Thornhill via Shirley, the city centre and Bitterne – replacing single-deckers.
The order represents an investment of £4m, and will help increase capacity on the route, and reduce emissions.
Bluestar Managing Director, Andrew Wickham, said: “This is in direct response to an increase in the number of passengers travelling along this popular route.
“We’re delighted more of our customers are choosing to leave their cars at home to travel with us on buses equipped with Euro VI low-emissions diesel engines and start/stop engine technology – that can only be a good thing for improving air quality in the city.”