Stagecoach's Megabus service, which offers fares from £1 or one dollar, is to ramp up its expansion by spending £44 million on more than 100 new buses.

Megabus's budget formula has proved a hit with travellers on both sides of the Atlantic since its launch with a few trial routes in Scotland in 2003.

The UK service carries around three million passengers a year, which Stagecoach hopes to improve on with the addition of 11 double-decker buses to replace single-deck versions currently operating on routes between London and Bristol, Cardiff, Sheffield, Leeds and Newcastle.

More than 15 million passengers have travelled with Megabus in North America since its launch in Chicago in 2006, enticed by the cheap fares and routes such as New York to Toronto and Chicago to Detroit.

The investment will bring 95 new buses to the US and Canada operation, expanding the fleet to 280 buses. There will be 87 buses in the UK fleet.

With greener engines and the convenience of toilets, wi-fi and power sockets, Stagecoach chief executive Sir Brian Souter said the service was attracting a customer who would have not used public transport in the past.

Sir Brian added: "Many of the people we are attracting, particularly in North America, used to travel by car, so the switch to greener coach travel is better for the planet."

Research by Stagecoach suggests 50% of passengers would have taken the car and 20% would have taken a flight, pointing to a new breed of customer - middle class and professional but thrifty at the same time.

Sir Brian has also identified another potential target market with the park and ride customer.

"That is opening up a whole new way of thinking about public transport in North America," he told the Financial Times recently.