Expected to create up to 400 jobs during construction, the power plants are planned to be operated as peaking plants to help ensure electricity supply when demand surges.
UK Energy Minister Lord Bourne said: "Gas is the greenest fossil fuel we have; generating electricity with only half the emissions produced by coal."
Hirwaun Power plans to construct a £200m gas-fired power station near Aberdare in south Wales by 2019.
Hirwaun Power project director Norman Campbell said: "Importantly, the project also helps to meet the government’s policy objectives of security of energy supply and affordability and supports the country’s transition to a low carbon economy."
Another £200m gas-fired power station project will be built by Progress Power near Eye in Suffolk, subject to raising the necessary finance. The power plant is planned to be commissioned in 2017.
Progress Power project director Chris McKerrow said: "We intend to work closely with Mid Suffolk District Council and Suffolk County Council to ensure that our project is developed sensitively, respecting the views of local people and carefully managing the project’s impact on the local environment."
