Employers with 50 or more employees must send forms P45, P46 and P46(Pen) to HMRC online from 6 April 2009.
Failure to do so could result in a penalty.
Forms are filed online using HMRC's PAYE online for employers service, which employers must first register with.
Once registered, forms can filed using HMRC's free 'online returns and forms - PAYE' service, commercial payroll software, or by electronic data interchange. Alternatively, employers can use a payroll agent or bureau.
Any stocks of old A5 versions of the P45 should be destroyed.
A new podcast, which explains the changes, and offers help and advice on preparing for them, is available to download free from the HMRE website's podcast section.