David Cameron has reneged on a pre-election promise to reward early adopters of solar panels and other domestic green energy generation, it has emerged.

Under the "feed-in tariff" scheme introduced in April, owners of solar panels fitted to houses after 15 July 2009 are paid 41.3p per unit of electricity, while householders who put up panels before that date get just 9p.

Green energy campaigners had fought the difference, which they called a "betrayal".

Responding in March to a letter from one of his Witney constituents calling for an increase in payments to such "pioneers", Cameron wrote in a letter seen by the Guardian: "I agree with you that the [Labour] government's current proposals for feed-in-tariffs will unfairly penalise the very people who were the early investors in local energy."

He added: "That is why under a Conservative government, any micro-generation technologies that have already been installed … will be eligible for the new higher tariffs once they commence."

Within days of taking power as PM, he also said the coalition would be the "greenest government ever".

But last month, responding to a question from Green MP Caroline Lucas the energy secretary, Chris Huhne, ruled out any such move. "I considered the issue carefully on a value-for-money basis, andI am afraid that the advice from my officials was clearly that we cannot introduce retrospection in such cases because it does not represent value for money," he said.

"We are trying to introduce new schemes in future, and therefore, sadly, the only incentive and payback that people such as the Hon Lady and I will get is the warm glow of being pioneers."

Charles Hendry, the Conservative MP who is now minister of state at the Department for Energy and Climate Change (DECC), also wrote pre-election letters to campaigners, saying a Tory government would pay higher rates to those who had installed early. Many Lib Dem MPs pledged it too, signing an early-day motion on the issue.

Julie Davenport, chief executive of renewable energy supplier Good Energy, said Cameron's U-turn showed he was failing on his promise of a "big society": "Good Energy is extremely disappointed that the government has not met its own pre-election promise to support early adopters of renewable technologies. We urge David Cameron to ensure that there is no further reduction to the feed-in tariffs in any way."

"Feed-in tariffs encourage new, local investment in green energy, are a catalyst to break up traditional energy markets and help many ordinary people make a difference to climate change. Isn't that what a "big society" really means – taking control from the few and giving it back to the many?"

In addition to Huhne's comments, recent speculation that the 41.3p feed-in-tariff rate will be cut as part of spending review annoucements on 20 October mean it is now highly unlikely that early adopters will get the redress they hoped for.

Fears that the feed-in tariff will be reduced or even axed for householders who since April have joined a gold rush of solar installs have led industry figures to write to Huhne, the business secretary, Vince Cable, and chief secretary to the treasury, Danny Alexander, warning of the risks of such a cut. When the subsidy was launched by Labour, the rate paid to microgenerators was set to be reviewed in 2012.

A group of 69 individuals and organisations, including E.on's Paul Golby, wrote last week that: "Such premature adjustments to the tariff would have a profoundly damaging effect on long-term investor confidence in the clean tech and renewable energy sectors, and may cause investors to flee altogether."