The Foreign Secretary yesterday told MPs and peers that breaking away would have "strategic and security implications" for each of the four nations.
He said Scotland would suffer most by terminating its links with the "most capable" diplomatic network on earth.
The potential problems have been discussed at length twice in recent months by the whole Cabinet, he told the Joint Committee on National Security Strategy.
Mr Hague added: ""It would be particularly damaging to the part of the country that was no longer part of the United Kingdom because here we are with that permanent membership of the (UN) Security Council, with 260 diplomatic posts around the world, one of the largest and most capable diplo- matic networks on earth and they would be excluding themselves from that."
He added: "Of course it would have strategic and security implications for the whole of the UK but I think it would be particularly damaging for Scotland."