The hole in the Ministry of Defence's budget is apparently £1.7bn a year, plus the annual cost of up to £500m a year of the UK's Continuous at Sea Nuclear Deterrent.
According to a senior government source, "without the cash this would mean cancelling significant conventional capability to affect our operational footprint and status".
Which is official-speak for "the UK's armed forces would be perilously hollowed out".
At risk of being cut, I am told, are purchases of US F-35 fighter jets, or the UK's amphibious capability, including 1000 marines or many thousands of members of the armed forces - with a risk that full-time army members would drop from around 80,000 to 58,000.
It is the increased threat from cyber warfare that is expected to absorb a growing share of the MoD's cash, according to a review of its resource needs.
That review will be completed in February.
The new and controversial Defence Secretary Gavin Williamson is "determined to seek a deal with the Treasury", said a minister.
But the Treasury claims to be broke.
On the other hand, around 30 Tory MPs, organised by the former soldier Johnny Mercer, are collectively putting pressure on the Chancellor to find the cash.
So this dispute is just one example of the vice that paralyses the Prime Minster - namely her government she has no money, and no parliamentary majority. If she spends on defence, police, doctors, nurses, firefighters, local government and so on one would legitimately ask "what about us?"
And if she doesn't spend her grip on office will be even more tenuous than it is now.
If Williamson can steer a path between this Scylla and Charybdis, his promotion from chief whip won't be seen as cynically by his peers as it is now.