UK says Northern Ireland can not be in separate customs territory post-Brexit

A sign for Blacklion village is seen on the Belcoo and Blacklion Bridge which served as a hard border between the border towns of Blacklion (Rep. of Ireland) and Belcoo (Northern Ireland) until 1998 is seen from Belcoo, Northern Ireland, July 4, 2016. REUTERS/Clodagh Kilcoyne

LONDON (Reuters) - Britain cannot accept Northern Ireland being in a separate customs territory from the rest of the United Kingdom, Prime Minister Theresa May's spokesman said on Tuesday.

Brexit talks have stalled over a disagreement on the so-called Northern Irish backstop, an insurance policy to ensure there will be no return to a hard border on the island of Ireland if a future trading relationship is not agreed in time.

Reuters reported on Monday the EU could look to "anchor" a reference in the legally binding Withdrawal Agreement to Britain's proposal to keep the whole of the United Kingdom in a customs partnership with the EU.

Asked whether Britain would be prepared to accept the Northern Ireland backstop if there were a link to a future treaty agreement on a UK-wide arrangement, the spokesman said: "Any circumstance in which Northern Ireland could be in a separate customs territory to the UK is unacceptable."

(Reporting by Kylie MacLellan; editing by Stephen Addison)