Shares in Centrica and SSE closed down by more than 1% after a promise by the Conservatives of a price cap for energy bills if they win the election.
Earlier British Gas owner Centrica was down 4% and SSE by 2%.
The other Big Six are not listed on the London market. Npower owner RWE and E.on are German, Scottish Power owner Iberdrola Spanish and EdF French.
The FTSE 100 share index made modest gains, up 41.35 points at 7,342.21.
Shares in Micro Focus International were down nearly 6% after the software firm said revenues at Hewlett Packard Enterprise’s software business, which it is taking over, had fallen by 10%.
Mining giant Glencore was up 2% after news it had started the sale process for its Tahmoor coking coal mine.
It plans to stop mining there next year. Other major miners joined the party with Anglo American and BHP Billiton both higher.
The pound was flat against the dollar at $1.2944 but up 0.35% against the euro at 1.1887 euros.
Brent crude prices were 0.55% lower at $48.79 per barrel.