Danny Alexander, the Liberal Democrat Treasury chief secretary, has accused his Conservative coalition partners of waging a "constant war of attrition" on green issues, warning that it is endangering billions in green investment, as well as the whole government growth strategy.

Alexander describes how the government is having to deal with Tory backbenchers – including those he calls "luddite" climate change deniers – opposing green technologies such as windfarms. "I just don't think the British economy can any more afford to have a blue roadblock to green growth," he says.

In a Guardian interview on the eve of the Lib Dem party conference, he also reveals a new fault line with his coalition partners, promising to oppose any Tory plan to freeze out-of-work benefits.

"A two-year freeze in benefits, which would freeze the incomes of the poorest people in this country, is not a place we should be looking at," he says.

He also insists that he does not accept George Osborne's suggestion that the next round of welfare cuts needs to be as high as £10bn. "I am not wedded to any number," he says.

Alexander's remarks represent a twin broadside against the chancellor, who has been urging further big welfare cuts as well as claiming that green policies will hinder growth if they make British industry less competitive

But Alexander does admit for the first time that the two parties will have to agree joint plans in a spending review next year for billions of so far unidentified Whitehall spending cuts to be implemented in 2015/16, the period straddling a June 2015 general election.

"The thing I will not do is sacrifice our party's independence by binding us to specifics on spending deep into the next parliament, but we definitely have to find those savings for the year 2015/16. Not to do so would be completely irresponsible", he says.

As chief secretary to the Treasury Alexander is at the heart of all budget negotiations across the government. He is a member of the "quad" at the apex of the coalition alongside David Cameron, Nick Clegg and Osborne.

He indicates that a new wealth tax could be agreed by the coalition before the election. He also gives a strong hint that the government may shelve its fiscal mandate – debt falling as a proportion of GDP in 2015/16 – if public finances deteriorate.

Alexander, normally the Liberal Democrat cabinet minister most loyal to the coalition, was speaking before a conference in which Nick Clegg will seek to re-energise the Lib Dems. Clegg is hoping his party's contribution to the coalition will get a more positive hearing now that he has apologised for pledging at the last election to oppose tuition fees.

Alexander marks out his clearest differences with the Conservatives on green issues. "The thing that in this job and in this government I find most frustrating is what often feels like a constant war of attrition on green issues, on renewable energy and renewable investment which is so important to our economy. I think that just has to come to an end.

"It seems to me that green investment is one of the fastest routes to getting investment, growth and jobs going in this country, and we just need to have a very, very clear position and a very, very clear plan. I don't think as a government we can afford to send mixed messages on that point."

The government, after a lengthy internal battle earlier this year, reduced subsidies for onshore wind power by 10% from April next year rather than the 25% reportedly sought by Osborne.

Alexander says he wants to establish a target range of 50g–100g of CO2 per kilowatt hour for the decarbonisation of the power sector by 2030.

But Cameron has appointed John Hayes, an opponent of windfarms, as the new energy minister, and the Treasury has been hinting that green subsidies should be dropped. Alexander says he will not comment on individuals but adds: "There are all sorts of noises off, particularly from Tory backbenchers about 'let's stop onshore windfarms, and massively cut back their subsidies' and it creates a lot of uncertainty for investors.

"They worry about the pressure going on amongst the backbenchers in one of the governing parties and what does it mean for the future.

"It means we have to deal with some of the luddites that continue to deny climate change and who think this is all a myth. I am not saying these views are held in government, but you do hear those noises in the wider debate, and the only way to deal with them is not for the government to pander to those ideas, but to get on and deliver our agenda."

Alexander rebuffs suggestions that the Treasury has become an institutional block to reform of social care by failing to implement proposals set out by the Dilnot commission.

He says: "I certainly want to see reform of social care. In the end these spending decisions are about choices. It is about what do you want to prioritise and where are you willing to make savings?"