"U.K. demand for funding from both businesses and households has been softening somewhat at the beginning of this year, which we believe is the first sign of the gradual slowing of the economy that we expect for 2017," said S&P Global Ratings Senior Economist Boris Glass.
In an accompanying press release, S&P noted that the Bank of England Credit Survey for the fourth quarter of 2016 painted a picture of diminishing corporate financing, despite ultra-easy lending terms.
And for consumers, S&P said robust yearly growth numbers masked some recent weakness.
"Net consumer credit actually slowed recently, notably in December and January, where seasonally adjusted unsecured lending to households was, at £1 billion ($1.22 billion) and £1.4 billion, significantly lower than the £1.6 billion average in the 11 months to November 2016," the report said.
S&P said it expected the Bank of England to "look through" inflation and "keep rates low for long, despite a material upward revision to growth."
Following the U.K. vote to exit the European Union, S&P downgraded the nation by two notches from "AAA" to "AA".
In this latest review, S&P noted that only a rating committee may determine a rating action and this report doesn't constitute any rating action.
The warning for the U.K. follows hot on the heels of Markit PMI data on Friday 3 March which suggested growth in the UK's service sector eased to a five-month low in February.
On that release, Chris Williamson of IHS Markit said the U.K. economy had "lost momentum" after "impressive growth" toward the end of 2016.