The US’s road safety regulator on Tuesday delivered its strongest public rebuke so far over General Motors’ handling of a botched ignition switch recall when it called the company’s failure to answer scores of questions promptly “deeply troubling”.

The National Highway Traffic Safety Administration said it was levying the maximum available fine – $7,000 per day – on GM for failing to provide answers to a third of the 107 questions it sent the company in March. The questionnaire was due to be completed by April 3.

The rebuke is the most significant setback so far for the attempts by Mary Barra, GM’s chief executive, to portray the ignition switch problems as confined to the “old GM” from before its 2009 bankruptcy. Ms Barra had vowed that the “new GM,” having discovered the previously undisclosed safety problem, would deal with it swiftly and transparently.