LONDON, April 24 (Reuters) - Shares in British energy providers Centrica and SSE fell by as much as 3 percent on Monday after Prime Minister Theresa May's Conservative Party vowed to cap domestic prices if it retains power in an election in June.
Minister Damian Green said on Sunday that the Conservative election manifesto would include measures on energy prices and the Sunday Times said the plans could cut gas and electricity costs by 100 pounds ($128)a year for 17 million families.
May's ruling party has previously said the energy market does not work properly and called for more competition in a sector dominated by the big six providers of Centrica, SSE, Scottish Power, Npower, E.ON and EDF.
The broad FTSE 100 index was up 1.9 percent on Monday in early trading. ($1 = 0.7822 pounds) (Reporting by Kate Holton; Editing by William Schomberg)