The US head of Volkswagen prepared to apologize to Congress on Thursday over the "deeply troubling" pollution scandal as it emerged he knew of a potential problem with vehicle emissions as early as spring 2014.

In testimony released ahead of his hearing with the House energy committee, Michael Horn offered a "sincere apology" for Volkswagen's use of a software program "that served to defeat the regular emissions testing regime".

He also admitted he was made aware of "a possible emissions noncompliance" in early 2014 after researchers at the University of West Virginia found that the cars affected released up to 40 times as much nitrogen oxide than was legally permissible.