Shares in power giants Centrica and Scottish and Southern Energy (SSE) fell shortly after markets opened today in the wake of Ed Miliband's pledge to freeze energy bills.
Centrica shares dropped 3.7% to £3.82, while Southern Electric and Swalec owner SSE fell 3.2% to £15.30 in a session where the FTSE 100 index opened down 0.2% at 6,561.
As Ed Miliband clashes with the energy giants on prices, it is worth knowing how your expensive energy bills are actually made up.
A leading competition lawyer says an energy price freeze could leave Labour open to challenges from the courts or the European Commission.
Whether or not you approve of a state intervention into industry, there are at least two big potential obstacles for an energy price freeze.