(updated) A row over energy policy has deepened after Nicola Sturgeon wrote to  Prime Minister David Cameron demanding he reassure Scots that the lights will not go out because of electricity supply shortages.

But her accusation that Westminster was responsible for forcing high transmission charges on Scotland was rejected by Labour MP Tom Docherty who leapt to the defence of the coalition and said it was the SNP’s push for renewables that was forcing up the price.

Speaking at a renewables event in Edinburgh (17th), Business Secretary Vince Cable said: “I don’t know why this has flared up. There is no threat to security of supply.”

In her letter to Downing Street, the First Minister,  called for a review of power supplies following concerns from energy experts that Scotland is paying a high price for selling electricity to the rest of the UK. It came amid speculation that the Longannet coal fired power station may close earlier than planned.

Dunfermline and West Fife MP Mr Docherty, however, said the Scottish government could not have it both ways and that renewables were even more costly. Longannet is in his constituency and he said he was prepared to defend the coalition because he felt the Scottish government was in the wrong on this issue.

Ms Sturgeon’s comments followed a meeting with the Scottish Energy Advisory Board which warned of looming problems for energy supplies.

Scotland is a substantial and reliable net exporter of power to the rest of the UK, with 28% of all Scottish generation exported in 2013, helping to keep the lights on and bills down across the whole of Britain.

Ms Sturgeon said in her note to the Prime Minister that Scotland’s energy security” is being compromised”.

She has called for an analysis of electricity capacity margins in Scotland and to seek reassurance that the UK Government “understands the key role which Scottish capacity plays in ensuring adequate levels of energy security across the UK”.

“It was clear from the meeting of the Scottish Energy Advisory Board today that industry experts are concerned about security of supply in Scotland and across the UK, and for the continued maintenance of electricity supplies in a robust manor. These issues need to be assessed and that is why I am calling on Prime Minister David Cameron to act,” she writes.

“It is vital that the UK Government fulfils its statutory responsibility to monitor security of supply issues and does so in an open and transparent way. The actions we have proposed would enable proper public scrutiny of the situation in Scotland.”

Scottish generators, including Longannet account for around 12% of the capacity connected to Britain’s high-voltage electricity network but pay around 35% of the charges.

Full text ofletter from the First Minister to Prime Minister David Cameron

16 February 2015

Dear David

The Scottish Energy Advisory Board, a panel of leading industry and academic experts that advise Scottish Ministers, met today to review the situation of security of electricity supply.

You will be aware the margin of spare capacity in the GB electricity system has diminished substantially in recent years to its lowest level in a generation, exerting upward pressure on the price of electricity.

Allowing the quantity of reserve energy to fall in this way is extraordinarily risky and fails the first duty of energy policy – to demonstrate that the system is secure and can meet consumers’ electricity needs at all times and under all circumstances.

Scotland is a substantial and reliable net exporter of power to the rest of the UK, with 28 per cent of all Scottish generation exported in 2013, helping to keep the lights on and bills down across these islands. Thermal generation plant in Scotland, including Longannet power station, plays a significant role in both delivering affordable electricity and system operability, as well as making a significant contribution to Scotland’s economy. The Scottish Government is clear that clean thermal generation will be required in the future as part of a diverse energy mix as we transition to a lower carbon economy.

However, our ability to continue to bolster the UK’s energy reserve is being undermined progressively by two factors: (i) high transmission charges that penalise generators based in Scotland and (ii) the absence of dedicated analysis and a separate reliability standard for Scotland.

To preserve viable options for the future, including the development of carbon capture and storage technology, we need to avoid the early closure of Scotland’s conventional thermal plant. Without careful management of existing capacity market dynamics could effectively close-off options that ought to be kept alive in the interests of system security, flexibility and energy affordability.

I am seeking your reassurance that the UK Government is fully cognisant of the unfolding capacity situation and understands the key role that Scottish capacity plays in ensuring adequate levels of energy security for the whole of the UK.

The Scottish Government cannot accept a situation where levels of energy security in Scotland are compromised by energy policy and network operation decisions taken outside Scotland. It is for this reason we ask the UK Government to initiate a dedicated capacity assessment for Scotland, informed by stakeholder views, and take steps to transfer to the Scottish Parliament the authority to set our own national reliability standard for electricity. Together these measures would help to create the conditions necessary to incentivise investment in new thermal capacity in Scotland.

In parallel with this letter, Energy Minister Fergus Ewing will seek urgent talks on Scottish energy security with Steve Holliday the Chief Executive of National Grid. I am copying this letter to SEAB’s co-chair, Professor Sir Jim McDonald, and the Coalition’s Energy Secretary, Ed Davey.

NICOLA STURGEON