The ex-Belgian prime Minister told the Observer that crashing out without a deal was “an existential threat to the UK itself”.
Scotland’s First Minister Nicola Sturgeon has previously said she will decide whether to call for a second Scottish independence referendum by the end of this year.
Van Rompuy believes a no-deal Brexit would have a “big impact” on “regions such as Scotland” and therefore aid the Scottish Government’s bid.
He added: “We could end up with a situation in which the EU27 becomes more united and a United Kingdom less united.
“This talk about a ‘no deal’ is the kind of nationalist rhetoric that belongs to another era.”