Ofgem outlined a series of measures designed to "break the stranglehold" of the six firms which dominate the UK market, and push down prices for hard-pressed households.
Energy Secretary Ed Davey said the Government was ready to bring in new laws should the new measures be "delayed or frustrated".
The regulator says it wants to create a more level playing field for independent rivals to buy and sell power. Smaller suppliers include the likes of Co-operative Energy, Ecotricity, Ebico and First Utility.
Under its plan the big six - British Gas, EDF, E.ON, NPower, Scottish Power and SSE - cannot refuse any reasonable requests from small suppliers to buy electricity. They have also been told they must sell power to the smaller companies at a fair price and negotiate fairly at all times. The big companies will be given deadlines for acknowledging and responding to requests.
Ofgem said the rules would also apply to the largest independent power generators, Drax Power and GDF Suez.
An obligation to post prices at which they buy power two years in advance would "make it far easier for independent suppliers to buy power for their customers", Ofgem said. The proposals come ahead of a statutory consultation this autumn, with the changes expected to come into effect next year.
Andrew Wright, senior partner for markets at Ofgem, said: "Ofgem's proposals will break the stranglehold of the big six in the retail market and create a more level playing field for independent suppliers, who will get a fair deal when they want to buy and sell power up to two years ahead."
Mr Davey said: "I encourage companies to work with Ofgem to implement these proposals as swiftly as possible. Government stands ready to use the Energy Bill to take necessary measures to improve energy market liquidity should Ofgem's proposals be delayed or frustrated."