If Labour’s 2017 manifesto is its leadership’s Bible, then its companion piece, the “grey book” of spending commitments, is the catechism. Signed off by John McDonnell, the shadow chancellor, the 8-page document sets out just how its £48.6 billion programme of new spending would be funded in practice and allowed the opposition to say its election pledges were fully costed.

Its contents could soon change quite significantly. The New Statesman has learned that the shadow cabinet has been asked to update their grey book costings in anticipation of a snap election. Labour sources stress that the point of the exercise is not to correct mistakes in the original - though its accuracy has been called into question - but to have a manifesto ready for an early poll.

The process is likely to spark an internal scrap. While some departments will have very little to do, any change to the costings of those with more expensive commitments will have serious ramifications for what Labour’s tax and spend commitments will look like.

Similarly, changes to accounting rules have the potential to expand or constrict the fiscal latitude available to McDonnell: last month, the Office for National Statistics ruled that the government must count a portion of student loan debt towards the national debt, which under Philip Hammond’s fiscal rules must be covered by extra tax revenue from somewhere.

His opposite number has more money to play with as a result. But with expensive questions like the shape of Labour’s policy on welfare unresolved, the political consequences are unlikely to be straightforward - and future rulings could have have the opposite effect. Even if, as is expected, the next manifesto looks much like the first, its birth could be very difficult indeed.