PERTH-based rail and bus group Stagecoach and City analysts have reacted sceptically to complaints that its Megabus coaches have broken weight rules, in the latest outbreak of hostilities in the Big Apple's transport market.

A study paid for by rival Adirondack Transit Lines suggested that a fully-loaded Megabus weighs 39,750lb –almost 4000lb above city and state limits for two-axle vehicles.

This has prompted a complaint being lodged by local politicians at the State of New York's Department of Transportation.

However, Stagecoach has insisted that it easily meets nationwide limits of 80,000lb.

A spokesman said: "Our vehicles are inspected and certified by the Department of Transportation.

"They are also fully compliant with federal regulations for motorcoaches.

"Megabus.com has been a huge success and is incredibly popular with passengers. It has helped spark a resurgence in inter-city coach travel as people switch from the car to a greener and better-value form of transport."

Adirondack, together with Aberdeen-based FirstGroup's Greyhound Lines and Peter Pan Bus Lines, are already disputing Megabus's right to pick up passengers outside Manhattan's Port Authority bus terminal.

Douglas McNeill, analyst at Charles Stanley, said: "It's hard to imagine that Megabus is guilty of such a basic oversight, and so this has the feel of a storm in a teacup."

Stagecoach's shares fell 5p or 1.9% 262p.