"It remains to be seen how far-reaching and comprehensive such a new agreement will turn out to be," Moody's lead sovereign analyst Kathrin Muehlbronner said.

"In our current baseline scenario we have indeed assumed that the UK should be able to secure an agreement that preserves many advantages of the UK's current Single Market access. We will see how likely such an agreement will be, once negotiations get under way with the EU in April," she added.

Moody's assigned a negative outlook to Britain's sovereign credit rating of Aa1 after the country voted to leave the EU in June, and in November said it was at risk of a further downgrade if it lost significant access to the EU's single market.

(Reporting by David Milliken; Editing by William Schomberg)