Legally-binding targets for green energy production in the UK may be "unrealistic", an influential committee of MPs has said in a report that is highly critical of the responsible department.
A "greater sense of urgency and purpose" is needed at the Department of Energy and Climate Change (Decc) to help meet targets which it agreed without "clear plans" of how they would be hit, the Public Accounts Committee (PAC) said.
Britain signed up to a legally binding European Union target to supply 15 per cent of all energy from renewable sources by 2020 when Labour leader Ed Miliband was energy and climate change secretary.
PAC chairwoman Margaret Hodge said progress on renewable energy targets had been "unacceptably slow" over the last decade, highlighting that the proportion of the UK's electricity from renewables rose from 2.7 per cent in 2000 to just 6.7 per cent last year - "well short" of a 10 per cent target set for 2010 but which will not be delivered until 2012.
That raised doubts about the UK's ability to hit the 2020 EU target, she said, while longer term carbon reduction goals were also in doubt.
"New, and substantially more demanding, targets are now in place," Ms Hodge said.
"The department will have to have a greater sense of urgency and purpose if it is to achieve the dramatic increase in renewable energy supplies needed to meet them.
"We are concerned that the department agreed to the legally binding EU target to supply 15 per cent of the UK's energy from renewable sources by 2020 without clear plans, targets for each renewable energy technology, estimates of funding required or understanding how the rate at which planning applications for onshore wind turbines were being rejected might affect progress.
"As for meeting the longer term 2050 target to reduce greenhouse gas emissions by 80 per cent, the department has yet to set out the timescale against which innovations in renewable energy technology will be required."
The report also found Decc does not know whether spending on renewable energy has achieved value for money, is taking too long to review subsidy rates and lacks a clear strategy for reducing carbon emissions.
More than £180m set aside to support renewable energy technologies was left unspent - described as a "wasted opportunity - and Decc does not control swathes of funding despite being responsible for meeting targets, the MPs said as they proposed a series of recommendations.
The MPs said Decc was relying on a "massive growth" in wind power and needs to take account of a 40 per cent attrition rate at the planning stage.
A renewable energy strategy was only published last year and work on a detailed delivery plan did not begin until January 2010, with publication not due until April next year.
The lack of a "coherent plan" meant the department "does not know whether value for money has been achieved from previous spending on renewable energy technologies".
The committee called on Decc to be more flexible with its review of subsidy rates, saying: "The department will need to act more quickly in response to changing circumstances, which may require it to move away from rigid review timetables that could result in delayed investment or increased costs for the bill payers who fund the subsidies."
And it urged the department, now led by Lib Dem Secretary of State Chris Huhne, to build up a clearer picture of what funding has been channelled through other bodies it does not control.
Tory committee member Matt Hancock (West Suffolk) said: "It is clear that over the past 10 years there has not been a consistent plan to tackle climate change
"Given the priority that everyone attaches to this, it is critical that the Government turns that around."