According to anti-GM lobby group Gene Ethics, in a letter dated February 26, three European grain traders told Mr Barnett they would not buy WA grain if GM canola is grown.

Gene Ethics claims that Barnett and Agriculture Minister Redman hid this letter from the Lower House during the disallowance debate on March 10.

"Redman failed in his duty to mention possible market losses despite asserting that markets are the state's primary responsibility," Gene Ethics director Bob Phelps said.

"They hid the letter which could have affected the vote that passed approval for GM canola by the slim margin of 26 votes to 24.

"They had a clear duty to disclose the letter's existence and its contents before the disallowance vote.

"When Legislative Councillors debate Giz Watson's disallowance motion and vote in the next few days, they should stand up for GM-free on the basis of the letter and the government's deviousness."

And in another letter to the Premier sent last week Japanese buyers have renewed their commitment to buy GM-free canola.

"The Japanese are writing GM-free contracts with South Australian growers, where the government has extended their GM canola ban till 2014," Mr Phelps said.

"In Victoria last season, farmers who produced GM-free canola were paid up to $15 per tonne more than farmers who grew GM.

"Elders Toepfer and CBH - buyers and marketers of bulk grains - both said they have markets for GM-free in Europe and Japan tht would not tolerate GM contaminated supplies.

"As a result, neither company would buy canola from any receival depot that handled GM.

"We call on the WA parliament to disallow the governments exemption from its GM-free Zone laws, so that commercial Roundup tolerant canola would continue to be prohibited as it is in South Australia and Tasmania."