George Osborne faces a black hole of more than £20 bn in the public finances,according to official government economic models. The news suggests that Britain might have to endure an extra year of austerity before the books are balanced. (Read more: UK's Osborne promises budget of 'hard truths' to balanceeconomy)

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The models by the Office for Budget Responsibility, which the Financial Times has replicated, indicate that the government should no longer rely on an economic recovery to eliminate part of the budget deficit. While Britain has staged a remarkable recovery, indicators of the economy's capacity for future growth have deteriorated. With unemployment falling quickly, the figures show that companies may have little room to expand production rapidly. More from the Financial Times:

OBR dilemma on message to Osborne

BoE holds rates and policy steady

Criticism of BoE policy seems unfounded The estimates come less than two weeks before Budget day and pose a difficult challenge to whichever party wins the election. If the models are correct, the next government would have to announce new spending cuts or tax rises to eliminate the structural deficit and ultimately to run a surplus.

The Conservatives have promised to eliminate the deficit by spending cuts alone: another £20bn of cuts would put serious pressure on welfare and "unprotected" budgets such as business support, local government and police. Mr Osborne, the chancellor, is already eyeing £12bn of new welfare savings.

Labour and the Liberal Democrats would both face the prospect of having to make deep spending cuts and put up taxes further than expected: both parties have promised to target the rich and expensive properties. In the run-up to the Budget the OBR faces a difficult choice in the message it has to give the chancellor. It could tell Mr Osborne he will need to pencil in more austerity: or it could tear up its own models, deeming them too pessimistic. (Read more: Summers and Osborne slug it out over austerity)

The black hole stems from the difference between the actual deficit –expected to be close to £111 bn in 2013-14 and the cyclically adjusted deficit estimated to be £85bn this financial year. So far politicians have assumed that they only need to look at the lower figure. Changes in the OBR's cyclically adjusted estimates have already been the primary cause of the government's extension of austerity policies from the five years planned in 2010 to the nine years currently thought needed.