I have read several times that the Labour rules prevent co-operation with other parties; it was claimed they cannot formally support a Parliamentary motion brought by another party, for example.

If this is a Labour rule, that does not mean it cannot be changed: there were the famous 'clause 4' debates, for example. But having had a quick scan of the rulebook, I cannot see such a rule in the first place. The nearest seems to be Chapter 13, clause X, but chapter 13 is all about local government, not the party as a whole.

Can anyone direct me to where the Labour rulebook prevents a formal co-operation with other parties?