The referendum on continued membership of the European Union is on June 23.
Spencer said it is important to realise that very few applications for Plant Variety Rights (PVR) now use the national PVR legislation of Member States.
Instead, breeders use the European Union Plant Variety Right administered by the Community Plant Variety Office (CPVO), which is an agency of the European Union, which simplifies forms, fees, examinations and legal issues, he said.
Spencer added: "It is important to note that, whilst applications for EU Plant Variety Rights from plant breeders outside the EU are welcomed, they must be placed through an agent domiciled within the territory of the European Union. However, the geographical scope in terms of enforceability of the EU PVR is limited to the territory of the Member States.
"Ever since EU PVRs were introduced in the 1990s, their scope has never been extended to any nation that was not a member of the EU. It is, therefore, reasonable to assume that if the United Kingdom were to leave the European Union, EU PVRs would no longer be enforceable in the United Kingdom."
Spencer said after Brexit, the UK and the EU could negotiate so that EU law regarding the PVR intellectual property right would continue to apply in the UK, but says "this outcome seems unlikely".
Alternatively, the UK could give a special exception to the requirement of UK PVR for novelty for a transitional period so that holders of existing EU PVRs could apply for UK rights, but UK Plant Variety Rights Office's (PVRO) £741 admin fee for the application itself, plus other fees means "this simply would not be worth doing" in many cases.
Spencer says the UK and EU could negotiate a situation whereby EU PVRs would continue to be valid in the UK. But, again, "this seems extremely unlikely" and that there would be "two parallel systems for PVR" with a UK PVR costing £2,221 and an EU PVR £1,590 according to Spencer's sums.
He says this extra cost could cost jobs, "stifle innovation in new plants" and mean increased royalties, and ultimately, more expensive plants.
Spencer concludes: "For plant breeders and innovators, we think that the argument is simple. Brexit can only make business more difficult and more expensive."