LONDON- The world's two biggest ratings agencies S&P Global and Moody's both warned Britain on Thursday that its credit rating would be under threat if a post-Brexit trade deal significantly cut access to European Union markets.

S&P rates the UK at AA stable having cut its prized triple-A grade by an unprecedented two notches in the days after the 2016 Brexit vote.

"Our ratings on the UK and our forecasts do not incorporate a rapid deterioration in the UK's terms of access to the EU. If this assumption does not hold true, downward pressure could build on the UK's sovereign ratings," two of its top analysts said in a report.

A separate Moody's report had a similar message.

"A looser economic relationship would result in structurally weaker economic fundamentals for the UK, and to a lesser extent the EU. This structural weakness is a clear credit negative for the UK sovereign and related issuers," it said.

(Reporting by Marc Jones; Editing by Tom Arnold) ((marc.jones@thomsonreuters.com; +44 (0)207 542 9033; Reuters Messaging: marc.jones.thomsonreuters.com@reuters.net Twitter @marcjonesrtrs))