Per Lord Reid in Koufos v C Czarnikow Ltd (The Heron II )  3 All ER 686 at p.690 and pp.694-695:
I use the words "not unlikely" as denoting a degree of probability considerably less than an even chance but nevertheless not very unusual and easily foreseeable.
It appears to me that in the ordinary use of language there is a wide gulf between saying that some event is not unlikely or quite likely to happen and saying merely that it is a serious possibility, a real danger, or on the cards. Suppose one takes a well-shuffled pack of cards, it is quite likely or not unlikely that the top card will prove to be a diamond: the odds are only three to one against; but most people would not say that it is quite likely to be the nine of diamonds for the odds are then fifty-one to one against. On the other hand I think that most people would say that there is a serious possibility or a real danger of its being turned up first and, of course, it is on the cards.