"We will continue to focus on the degree to which the Brexit process risks damaging UK medium-term economic prospects and public finances," Fitch said in a statement in response to a major speech from May on Tuesday.
"The range of possible Brexit outcomes remains wide and the envisaged timescale for reaching a UK-EU trade agreement ambitious," it added.
May hopes to reach a trade agreement during the two years of formal talks to leave the EU which she plans to start within two or three months.
Fitch downgraded Britain's credit rating to 'AA' with a negative outlook after June's Brexit vote.
(Reporting by David Milliken; Editing by William Schomberg)
