Vince Cable has demanded that the Office for Budget Responsibility spell out the difference between Tory and Liberal Democrat economic plans amid concerns that the Treasury has not been honest enough about the impact of future spending cuts.

The extraordinary intervention – emerging in the aftermath of the autumn statement given by George Osborne – represents an attempt by the business secretary to assert the independence of Liberal Democrat economic policymaking after more than four years of coalition government.

Cable wrote to the OBR in the past two weeks; his letter as well as a reply from the OBR is due to be published shortly.

It is understood Cable asked the government’s fiscal watchdog to make sure it spelled out what was agreed coalition economic policy and what was going to be implied by the OBR in its forecasts about future spending and cuts after the election.

He also wanted to make sure the OBR projections show that the two coalition parties had very different approaches to raising taxes as part of the fiscal consolidation, and not to assume the two government parties would use the same mix of tax and spending cuts to achieve a fiscal surplus.

The OBR confirmed it had received Cable’s letter and pointed to aspects of the report it released after Osborne finished his statement to the house that had tried to show the uncertainties due to the differences between the two parties.

Cable’s call for greater public transparency in the national budget-setting process reflects the tensions between the two coalition parties over economic policy and some of the differences of emphasis within the Liberal Democrats.

The business secretary is known to be concerned that it will not be possible to achieve the spending cuts required to erase the deficit by 2017-18, as Osborne proposes, without causing severe damage to departmental spending.

The OBR shows spending by government departments per head will have fallen from £5,650 in 2009/10 to £3,880 in 2019/20 as measured in 2014/15 prices. Public spending is projected to fall to its lowest level as a proportion of GDP since the 1930s.

Cable has been privately examining the impact of the required spending cuts on unprotected departments such as defence and local government for weeks and thinks the coalition leadership have not been open about what it might mean for those ministries, including his own business department.

Danny Alexander, the Liberal Democrat chief secretary, on Tuesday said only small cuts were required in the next parliament, adding that three-quarters to two-thirds of the spending cuts had already been secured in the first parliament. However, such claims, also made by the prime minister at the Conservative conference, have twice been challenged by the Office for National Statistics.

The OBR insists that 60% of the spending cuts have not yet been implemented in this parliament, but Conservative Treasury figures said the majority of cuts had been legislated for.

The senior Lib Dem is determined that the election campaign debate is held on the basis of an accurate assessment of the impact of unprotected departments, which also include the Home Office, the Ministry of Defence, the Foreign Office and the department of communities. He was determined to ensure the OBR figures were presented to show the options if the cuts continued at the current pace with some departments are protected.

Cable played the role of Liberal Democrat economics spokesman in the 2010 election, but has been overlooked by Nick Clegg in favour of Alexander, who has been at the centre of the party’s economics debate.

It is also known that Cable has been unhappy that some civil servants have been asked by the Treasury to start a preliminary examination of potential spending cuts after 2015-16.

He has ordered his civil servants not to participate, arguing that it is not right that there is a secret spending review under way before the electorate has delivered its judgment.

Some ministries have been exempt from spending cuts. Under the coalition, health, education and international development were ringfenced. David Cameron has promised to protect health in the next parliament if the Conservatives win the next general election.