Vince Cable has reportedly written to the OBR demanding they spell out the differences between Lib Dem and Conservative policy over how to make the spending cuts needed in the next parliament.

If you look at this graphic – whose information is taken from the OBR’s report – you can see why he might think the current projections unachievable.

Because the government ringfences spending on schools, health and aid, other departments bear the brunt of cuts.

In 2009/10 – in today’s money – the combined spending on those other departments was £188bn. Today it’s fallen to just £147bn.

But the OBR says 60 per cent of the money to be cut lies ahead – and on that calculation, if we go on protecting health, schools and education, the money available to the rest of Whitehall will be just £86bn.

The IPPR think tank has crunched the numbers and we can reveal their findings exclusively here.

Their calculation says that, using 2016 money, the combined departmental cuts needed would be £54bn over the next parliament.

£4.2bn from the non-schools part of the education budget

£4.7bn from business and skills

£3.6bn from the Home Office

and more than £9bn from defence

Because you can’t calculate the Barnett formula, which allocates taxation to Scotland, Wales and Northern Ireland, you can’t put numbers on their cuts but my estimate, using the same methodology, would be a something like £18bn.

We will end up with a state much smaller than at any time in the past 80 years.

The big question is can services and institutions that have withstood 40 per cent of the cuts so far withstand the other 60 per cent.

The scale of these cuts is the prime reason why no politician – Labour included – wants to spell them out.

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