Perth based Stagecoach said it is spending £102m on new fleet this year which will provide it with close to 570 additional vehicles to put on the road.
Within that Alexander Dennis, which is understood to have grown its turnover by around 10% to £530m in 2013, will build more than 470 vehicles over the next 12 months including double deck, single deck, midi and hybrid-electric buses and eight luxury coaches.
Although no new jobs will be created the large order is likely to support the jobs of the 900 Alexander Dennis staff in Falkirk as well as hundreds more at facilities in Scarborough and Guildford.
Alexander Dennis chief executive Colin Robertson said: "This is another massive boost for the whole of the bus manufacturing sector and from [our] perspective I am delighted that we have again won the lion's share of the new business.
"We have worked closely with Stagecoach over many years to ensure that we design, engineer and build class-leading products that set the pace in terms of fuel economy, passenger comfort and operator benefits, and this order is a further endorsement of our market-leading product range."
Stagecoach's London operations are getting a £21m fleet of hybrid buses with the remainder of the new fleet being put into use outside the English capital.
That includes 17 new megabus coaches and three sleeper fitted megabus gold vehicles for its low cost intercity service.
Other companies which won orders were Belgian company Van Hool, Leeds based Optare and Northern Ireland's Wrightbus.
Chief executive Martin Griffiths said: "This record investment is a major boost for UK jobs in manufacturing and many other smaller businesses in the supply chain.
"It is also a huge vote of confidence in the future of bus and coach travel in Britain."