The past 12 months will come to be seen as "the year when the coalition Government lost the benefit of the doubt", a senior shadow cabinet minister has said.

Shadow foreign secretary Douglas Alexander said 2012 was the year when it became clear that Chancellor George Osborne's deficit reduction policy had "failed" and when voters decided that the Conservative Party led by David Cameron were "just the same old Tories".

Alexander credited the Government's failings for much of Labour's surge in the opinion polls over the last year, but insisted Ed Miliband's party was not "complacent" and recognised there was no guarantee it will win the general election scheduled for 2015.

He said Labour will flesh out a range of policies during the next two years and give detail on its tax and spending plans in time for the election manifesto.

Meanwhile, senior Blairite Gisela Stuart warned that Labour needs to be "much clearer" about what it would do in government and admit that it would have to make "harsh cuts" in some areas.

Alexander told BBC Radio 4's World at One: "I think 2012 will be seen as the year when the coalition Government lost the benefit of the doubt, not simply in terms of the accumulating evidence that in terms of its own test of deficit reduction it has struggled and failed, but also that it has lost the benefit of the doubt on motive.

"Up until the Budget last March, people were in significant numbers willing to think this was a Government that was reluctant to undertake the changes that were being made, was trying to govern in the national interest and faced some difficult choices.

"I think that with the decision to cut the top rate of tax and the decision to impoverish some of the poorest people in society, actually a much more familiar characterisation returned, which is that these are just the same old Tories.

"That explains why for the last nine months we have seen the Conservatives struggling so badly and Labour, as a consequence, benefiting in the public's mind."

Alexander said Labour had learned from its prolonged slump in the 1980s that it must provide a "credible not complacent" opposition and must spell out its programme for government in the coming years if it hopes to return to power.

"The truth is that David Cameron's failure on the economy is an opportunity for us to win the next election, but it is certainly not a guarantee that we will win the next election," he said.

"That's why in 2013 and 2014 we will of course provide more detail as our thinking develops and as the picture of the economy at the time of the next election becomes clearer. We will set out our thinking both in terms of spending and in terms of taxation at the time of the manifesto."

Stuart, who backed David Miliband for leader in 2010, said that the Parliamentary Labour Party as a body had had to "get over the fact that as a group we would have collectively preferred David over Ed".

But she said the younger Miliband brother had impressed his MPs with his "steeliness" on taking office.

"Ed started his leadership with some very shrewd internal moves which I don't think were noticed outside," she told World at One. "The way he asserted his right to choose the chief whip, the way he told the PLP 'I choose my shadow cabinet'. The PLP started to get a sense of his steeliness, which isn't terribly obvious."

The Birmingham Edgbaston MP added: "As we go towards the election, we will have to be much clearer as to what an alternative Labour government will look like and that will have to spell out some of the harsh cuts and things that we will no longer do."

Alexander said Labour's policy agenda would become clearer following the completion of the review being undertaken by former deputy leadership challenger Jon Cruddas.

But he said that, under Miliband, the economy would look "fundamentally different" to how it has been run under the coalition and by Labour administrations before the crash of 2007/08, with less reliance on the financial sector.