LONDON (AP) — Credit ratings agency Moody's is warning that it will downgrade Britain's rating if the country fails to get a good-enough deal on access to the European single market during upcoming negotiations to leave the European Union.

More than four months after Britain voted to leave the EU, the terms of Brexit remain to be ironed out. Prime Minister Theresa May has said she will invoke Article 50 of the EU Treaty by the end of March. That will launch two years of talks on Britain's exit.

Kathrin Muehlbronner, a Moody's senior vice president, said Wednesday that the agency would downgrade the U.K.'s near-top Aa1 rating, if it considers any loss of access to the single market "would materially damage its medium-term growth prospects."