The order will provide a boost for Falkirk-based Alexander Dennis, which will build around 100 of the 425 buses concerned.
But Wrightbus of Northern Ireland will build 300 vehicles. These will include 274 small buses featuring hybrid systems that FirstGroup expects will increase fuel efficiency by around 10%.
Aberdeen-based FirstGroup said it worked with Wrightbus throughout 2013 to develop the Mycro Hybrid buses.
Asked whether the decision to award the Micro Hyrbid business to Wrightbus was a snub to Alexander Dennis (ADL), a spokesman for FirstGroup said: "We enjoy a great relationship with ADL, who remain one of our preferred and most valued suppliers."
Alexander Dennis won orders for 222 vehicles from FirstGroup in the latest financial year, around half the total bought by the bus and trains group.
Bill Simpson, group corporate affairs director of Alexander Dennis, welcomed the latest order. The company will build 60 double deckers and 37 single tier vehicles.
It employs 900 people in Falkirk.
He noted: "We have got a strong order book taking us into 2014 and continue to be the market leader and to have market leading products in every sector."
Optare and Volvo also won orders from FirstGroup in the latest round.
On Wednesday FirstGroup faced renewed calls to separate its US and UK operations from Sandell Asset Management, a 3.1% shareholder.