Shares in Centrica, which owns British Gas, tumbled 5.15 per cent at 197p while SSE's stock fell 3.67 per cent at 1,392p in morning trading.
The Conservative Party has plans to put a cap on household energy bills, one of the UK's biggest providers has said this could lead to less benefits for the consumer.
On Sunday, Labour promised to introduce four new public holidays if it wins the June election, marking the saints days of England, Scotland, Wales and Northern Ireland.+ Leftist leader Jeremy Corbyn said the measure would "give workers the break they deserve", while also uniting the four nations, whose ties have been strained by the vote to leave the EU.
The company recorded pre-tax profits of £2.2billion previous year.
But Scottish Power, one of the "Big Six" energy firms, told the BBC that the move would "stop competition".
The energy industry has doubted the new plan and said a price cap could lead to higher prices.
'A price cap would be the death knell for competition. He said the energy regulator would set a cap that could reflect market conditions, and that it would work differently from the Labour policy.
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"On a positive note, British Gas" standard variable tariff is very competitive versus peers, thus we would not expect the full £100 per customer impact, ' he said, adding the hit would be around half that at £50.
Ofgem would play a major part, setting a maximum price for energy bills in different regions.
About 800,000 of the poorest pensioners and 1.5 million low-income families with children are on standard variable tariffs, according to Citizens Advice.
Share prices of listed energy companies fell on Monday after the Tories made the announcement ahead of the June 8 General Election.
Mr Green told the Peston on Sunday programme: "There will be a lot about energy policy in the manifesto [and] obviously there will be more detail".
"Intervention on this scale will additionally create huge uncertainty around government intentions, potentially putting at risk the billions in investment and jobs needed to renew our energy system", he said.
Richard Neudegg, head of regulation at comparison website uSwitch, said: 'Quite rightly, politicians want to ensure that consumers get a better deal on energy. It would remove any incentive for energy companies to drive down prices and fight to keep their customers, entrenching the position of the incumbent big six.
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