The chancellor has defended his decision not to spell out which departments will face the sharpest spending cuts in the years ahead.

In yesterday’s pre-Budget report Alistair Darling said that setting out departmental spending limits for the years beyond 2011 was “neither necessary or sensible”.

Speaking to the BBC, Darling defended the move, which has been criticised by opposition politicians and prompted Whitehall speculation over where the spending axe will fall.

“We have deliberately not set the budgets for individual departments [beyond 2011],” Darling said. “We are not in a position to say ‘this is the budget for the department of business, this is the budget for the department of agriculture’. I can’t do that.”

But the chancellor did say the government was assuming that spending on “non-protected areas” – defined as “frontline” health, education and police numbers – would remain “flat” in the years following 2011.

He also said that the possibility of lower unemployment could mean more resources being available to departments than currently predicted. “It could well be that if unemployment turns out to be lower than it was, we can put that money towards [departmental budgets].”

And Darling appeared to raise the possibility of departments being reallocated some money saved through the efficiency savings and pay freezes outlined in this week’s Smarter Government policy paper. “What I’m saying to departments is if you want to maintain spending, if you want to look after some areas more than others, there will be an opportunity for that reallocation to make more funds available.”

But shadow chancellor George Osborne insisted that the government’s failure to provide details of spending cuts beyond 2011 as evidence of its “ducking” the tough questions.