New Zealanders are being asked whether the Government should allow nuclear-powered ships back into our waters. Since the 1980s prime minister David Lange banned nuclear weapons and nuclear power, sparking a diplomatic stand-off with the US that has never been resolved, the nuclear ship issue has become a sacrosanct touchstone of New Zealand foreign policy.

But now, workers for a market research company have disclosed they have been polling voters about nuclear power. The workers have been in a pay dispute with their employer, Oceania Customer Interaction Service (OCIS).

The Unite union last night settled its dispute with the company, but not before claiming the survey was being carried out for the National Party.

The National Party has denied it was involved. A spokesman for Prime Minister John Key said: "We've never heard of the company, and we're not using them." He said National had no intention of changing the anti-nuclear policy.

Unite boss Mike Treen said members at the North Shore-based OCIS call centre had last month conducted a short survey about attitudes towards dropping the ban on nuclear-powered vessels. He said staff believed the client was the National Party.

In the past, both National and the Act Party have supported a public referendum on the ban, but National has since backed away from even considering a change to the law.

ACT leader Rodney Hide said he had no knowledge of the research, and was surprised to learn it was being conducted. "I can't imagine who it is. No one has said anything to me at any stage."

Right-leaning think-tanks, the Maxim Institute and the Business Roundtable, also denied knowledge of the survey.

Don Brash, the former National leader, said his leadership was adversely affected when he dared to discuss the ship ban. He said he would not change the law without a referendum - but that was publicly interpreted as a willingness to consider change.

"It's always been a very sensitive issue in New Zealand, and I can understand why politicians don't want to go anywhere near it," he said last night.