LONDON — Party loyalists applauded. Investors blanched.

Shares of British electric utility companies fell sharply Wednesday, a day after the head of the opposition Labour party, Ed Miliband, vowed that if his party won the 2015 elections the government would “freeze gas and electric prices until the start of 2017.”

The stock of the utility SSE lost 6 percent of its value Wednesday. Shares of another, Centrica, dropped by 5 percent, in a pattern that played out across the industry.

Mr. Miliband, addressing the Labour Party conference, was reacting to growing public concern about rising energy bills. But investors are concerned that any curbs on utility rates would further diminish the companies’ readiness to invest in new power capacity.

“It is political posturing,” said John Musk, an analyst at RBC Capital Markets in London. “It is hard to see how what he is suggesting will be workable.”