Nuclear giant British Energy has calculated that even if the SNP administration achieves its goal of generating 50% of electricity from renewable sources – such as wind or tidal power – by 2020, Scotland will need another four gigawatts to meet demand.

With the planned closure of five ageing nuclear, gas and coal-fired stations, this is equivalent to building three extra power plants by 2025.

If it fails to meet the shortfall, Scotland risks being forced to buy electricity from England and Northern Ireland, exposing Scottish households and businesses to volatile global commodity prices.

Robert Armour, general counsel and company secretary of British Energy, will tell ministers at a conference on Scotland's energy future on Tuesday that if they want Scotland to continue to be self-sufficient in electricity and guarantee the lowest prices for consumers, they will have to rethink their energy plans, in particular Salmond's determination to stamp out nuclear electricity generation by 2020.

He will warn that renewables alone will not be enough and the administration risks shooting itself in the foot by importing electricity from south of the border, as imports are likely to come from nuclear, gas or coal-fired plants anyway.

He will say: "Scotland is doing the right thing by setting challenging targets for CO2 reduction and efficiency. But even if we meet those targets, we still have to face up to replacing our base-load power stations because renewables and efficiency on their own can't provide the full answer.

"We will need the equivalent of three major power stations in addition to renewables by 2025. If there is a significant move to electric vehicles, that will put even more pressure on demand. The question we face is how to achieve a robust supply that insulates the Scottish economy from volatile commodity prices and delivers a low-carbon future."

British Energy will urge Salmond and Scottish Energy Minister Jim Mather to give the go-ahead to three new traditional power stations. Armour will argue that if they operate at full capacity alongside renewables, they would allow Scotland to maintain its current position as an electricity exporter, giving the country an economic boost.

British Energy's recommendations were backed last night by the conference organisers, the Scottish Council for Development and Industry, which is urging Salmond to re-think his no-nuclear policy in particular.

Lesley Sawers, chief executive of the SCDI, pointed out that nuclear energy generation will help the Government reach its goals on cutting carbon dioxide emissions. She said: "Secure, affordable and reliable energy is absolutely vital to economic development and continued business growth. Business and the unions share concerns about the Scottish Government's no-nuclear policy, and the implications this could have for cost, security of supply and Scotland."

Lewis MacDonald, Labour's energy spokesman in Scotland, welcomed British Energy's findings. "There is a very clear energy gap. Even with the most ambitious renewables target met, it doesn't do the trick. The reality is Scotland has a very mixed energy portfolio… and you can't replace oil, gas and nuclear all at once. Even the Council of Economic Advisers is saying renewables won't be enough."

But the Scottish Government last night dismissed the research as "nonsense". A spokesman said: "These claims – by a company that generates nuclear power – are clearly nonsense. They are based on existing power stations closing and no new commercial investment – which is the exact opposite of what is happening in Scotland.

"There is strong commercial interest and activity in investing in massive new generation capacity in Scotland. The claims also ignore Scotland's vast clean, green energy potential. Harnessing that potential can meet our future energy demands many times over, while also tackling climate change, and avoid both the radioactive waste and enormous decommissioning costs that are part and parcel of nuclear power."