Nearly two in five oil and gas firms have said that the proposed 2014 referendum on Scottish independence is a factor in their planning.

The results of a closely watched survey show possible constitutional and regulatory reforms have joined the list of issues exercising firms, in an industry in which participants prize stability but are grappling with changes on several fronts.

In the latest oil and gas survey for Aberdeen & Grampian Chamber of Commerce, the sponsors Bond Pearce noted: "This survey has ... highlighted a new concern with 39% of respondents indicating that the planned referendum in Scotland in 2014, and its possible consequences, is a factor in future plans and investment proposals."

The majority – 56% – of respondents said the referendum was not a factor, and 5% did not answer. Included in the survey for the first time, the question did not require respondents to elaborate.

A Scottish National Party MSP, Maureen Watt, said the report showed that most of the companies did not think the 2014 referendum was a factor in investment in the North Sea industry.

Bond Pearce said the authors hoped to explore the potential impact of the referendum further in future surveys.

Paul Stockley, joint head of oil and gas at the law firm, noted the survey also highlighted significant concern at the European Commission's proposal to take over the regulation of offshore oil and gas safety.

Claiming there was an "unacceptably high" risk of a major offshore accident in European waters, the EC wants to centralise control of health and safety and environmental protection in Europe. Currently national governments regulate activities in their own waters.

The proposal has been criticised by the industry body Oil & Gas UK. It has noted that the EC says the UK is in a group of North Sea countries that have world-class risk-based regulatory systems.

Mr Stockley said: "To wrestle control from the UK, a world leader with unparalleled experience, and place it the hands of the EU, in which only a handful of member states have any oil and gas, could be like setting a ticking time bomb."

Aberdeen & Grampian Chamber of Commerce said the survey, conducted by the Fraser of Allander Institute, showed the oil and gas sector continued to outperform the rest of the Scottish economy. Demand for employment remains strong.

"The outlook for the UK Continental Shelf (UKCS) remains positive and the latest survey indicates increasing activity," it said. But confidence in international markets remains much higher than in the UKCS.

Bob Collier, chief executive of Aberdeen & Grampian Chamber of Commerce, said: "Optimism in the sector is strong and the overall outlook ... is good, but this will only be realised if the right environment is put in place to allow the sector to flourish by removing the barriers to growth and building policy stability."

While respondents welcomed the concessions the Chancellor included in the latest Budget, the chamber said some still felt concerns about the stability of the UK fiscal regime that were triggered by last year's tax hike.

Respondents reiterated concerns about skills shortages.