From the judgment of Lord Denning, M.R. in Federal Commerce & Navigation Co. Ltd. v Molena Alpha Inc. and Others,  2 Lloyd’s Rep. 132 at p. 140, col. 1;  1 Q.B. 927 at pp. 974-975:
But one thing is quite clear: it is not every cross-claim which can be deducted. It is only cross-claims that arise out of the same transaction or are closely connected with it. And it is only cross-claims which go directly to impeach the plaintiff’s demands, that is, so closely connected with his demands that it would be manifestly unjust to allow him to enforce payment without taking into account the cross-claim.