SINGAPORE, May 9 -- Four Singapore-based tour bus companies have been suspended from operating in Malaysia after authorities found they had breached cross-border transport regulations.

Malaysia's Land Public Transport Agency (APAD) said the companies had their permits revoked after they were found offering unauthorised express bus services and selling tickets online - activities prohibited for foreign operators.

Under Malaysian rules, foreign-registered buses are allowed to bring tourists into Peninsular Malaysia only for overnight stays. However, they are not allowed to operate as express services, nor to sell individual tickets online. Operators must also submit passenger lists and detailed travel itineraries, with all trips beginning...