Around 30 percent of public buses checked this year in Hanoi emit black smoke, badly polluting the environment.
Trinh Ngoc Giao, head of Vietnam Register Department, told news website VnExpress that the agency in 2010 checked public buses five times at random at the end of several routes, and found a large number failing technical standards.
The department has withdrawn the registration papers of the failing buses and will only return them once the buses have been repaired, he said.
Giao said the buses were emitting black smoke as they were poor quality imports from China that had not been maintained regularly.
Also, the buses were frequently overloaded during peak times, with 50 or 60-seat buses carrying more than 100 people, he said.