Employer Provided Company Cars
Historically, when employers started to provide, changed or stopped providing cars to employees, Form P46 (Car) had to be submitted by the employer to HMRC. This enabled HMRC to keep tax codes up to date, which in turn ensured that the employee didn't end up with an unexpected tax bill.
From April 2009 employers were told that they no longer had to submit Form P46 (Car) for replacement company car changes. This was then followed by a statement that, from April 2010, Forms P46 (Car) relating to replacement company car changes would not be accepted.
However, this can lead to problems with tax codes and tax arrears. Following discussions with employers, from April 2011 employers will be able to notify HMRC electronically of any replacement car changes but paper forms will still not be accepted.
Link: HMRC statement