The British Horseracing Authority (BHA) has modified the Rules of Racing Rule 144 that pertains to Veterinary Certification with regard to non-runners. This modification comes into force on the 22nd March 2008.
 
As from that date, a racehorse trainer will be able to self-certify a horse he/she considers unfit to race as being a non-runner. In such an event, the horse will not be permitted to race for a period of 6 days from date of self-certification. Veterinary certificates issued as a result of a clinical examination by a veterinary surgeon will NOT be subject to the six day restriction, but cannot rescind a trainer's self-certification.
 
The full amended Rule 144 and Instruction D10 and the new Veterinary Certificate can be seen under the section ‘ forms to download’ on the right.

In essence, the difference between the new Veterinary Certificate and the one it supersedes on the 22nd March is marginal. With trainer self-certification, there will be no need for an Option B on a Veterinary Certificate (which stated "you could find no abnormal clinical findings but the trainer or his/her representative reported the following").
 
For full details please see the BHA’s website:
http://www.britishhorseracing.com/inside_horseracing/about/whatwedo/Veterinary/default.asp