Labour has accused Danny Alexander, Chief Secretary to the Treasury, of being "thick as thieves" with the Conservatives - despite "pretend" attempts to distance himself from his "consistent record of Tory collaboration".

Shadow treasury minister Chris Leslie asked his opposite number, who was standing in for Chancellor George Osborne in the Commons, whether he remained supportive of last week's Autumn Statement.

During Treasury questions, Mr Leslie asked Mr Alexander whether he stood by the estimated drop in total managed expenditure to 35% of GDP - a level not seen since the 1930s - by 2020.

The Liberal Democrat frontbencher replied: "The way the Autumn Statement is constructed is the Office for Budget Responsibility (OBR) give an assumption about the path of public finances over the course of the whole of the next parliament.

"Post 2017/18 there is a neutral assumption built into the public finances which assumes the public finance spending stays flat in real terms.

"That is something that enables the OBR to construct their forecast. In my view, post 2017/18 when we have finished dealing with the structural deficit, public expenditure can grow faster than that."

Mr Leslie then asked: "Why are you pretending now to distance yourself from your consistent record of Tory collaboration when you have been thick as thieves with them vote after vote, year after year, time and time again?"

Mr Alexander hit back saying it must be a "tough job" being shadow chief secretary to the Treasury as it meant having to deal with shadow chancellor Ed Balls, who Labour former chancellor Alistair Darling has described as a "difficult man".

He went on: "I guess that is what they mean by a zero-zero economy: one Ed has zero influence, the other has zero credibility.

"Let me say to the party opposite and the party behind me - both of them in different ways are advocating relentless austerity for the whole of the next parliament.

"It is only the Liberal Democrats turning round the public finances after 2017/18 that offer any hope of a change in the future."