Act Party leader John Banks will make a rare vote against his coalition partner National this week to maintain his passionate defence of animal rights.
The Psychoactive Substances Bill is expected to return to Parliament tomorrow and the Government could come close to losing a vote on a Green Party amendment which would ban animal testing for the purposes of approving legal highs for sale.
The bill states that animal testing should not happen if alternatives exist, but some MPs want the tests ruled out.
A proposed ban on animal experiments tabled by Green MP Mojo Mathers has the support of Mr Banks, who told Parliament on Wednesday: "Protecting animals is ingrained in my soul."
Mr Banks said the bill was well-intentioned but he could not support it in its present form.