The US wants Toyota to pay a record fine - it has two weeks to appeal
The US transport department is asking Toyota to pay a record fine of $16.4m (£10.7m) for holding back information about faulty accelerator pedals.
The department says the company failed to notify it about the flaws "in a timely way".
The National Highways Traffic Safety Administration (NHTSA) said documents provided by Toyota showed the carmaker knew about the defect in September.
Reports of problems with the pedals prompted a massive recall in January.
Toyota has two weeks to appeal against this penalty.
A statement on Toyota's website said it had not yet received a letter on the matter from the NHTSA.
The statement went on to say: "We have already taken a number of important steps to improve our communications with regulators and customers on safety-related matters as part of our strengthened overall commitment to quality assurance.
"These include the appointment of a new Chief Quality Officer for North America and a greater role for the region in making safety-related decisions."
The January recall was one of three the Japanese car company has been forced to make - others related to floor pedals that stuck in mats and braking issues.
Toyota ended up recalling 2.3 million vehicles in the US over this fault - as part of a total of more than eight million vehicles recalled worldwide relating to the three faults.
The NHTSA said Toyota issued repair procedures in late September to distributors in Europe and Canada to deal with complaints of sticking pedals but did not report the problem to it.
The country's Transportation Secretary, Ray LaHood, said in a statement: "We now have proof that Toyota failed to live up to its legal obligations."
He added: "Worse yet, they knowingly hid a dangerous defect for months from US officials and did not take action to protect millions of drivers and their families."
US safety regulators are still examining whether Toyota has committed other violations.
Toyota is in the process of fixing the fault by either replacing the pedal completely or by adding in a metal "shim".
Latest figures from the US showed the carmaker's sales bounced back in March - with hefty discounts helping to win back customers who had been shaken by the firm's mass safety recalls.