Standard and Poor's (S&P) on Friday retained the UK's sovereign credit rating steady at double-A. It, however, kept the country on a negative outlook.

This meant that risks of a further downgrade remained. The credit rating agency said this was amid an ongoing uncertainty about Brexit. The UK used to earlier enjoy a triple-A credit rating. However, post the 23 June referendum vote, this was downgraded by two notches to the current AA.

S&P explained that the UK's vote to leave the European Union in June had created "a less predictable and stable policy framework". It added that this had created doubts on how the UK economy would perform once it is out of the EU.

"In our opinion, Brexit presents a significant risk to the UK's track record of strong economic performance, and to its large financial sector in particular," the agency said. This follows the UK reporting a better than expected GDP on Thursday. It said that its economy had grown by 0.5% over the third quarter of 2016, in the three months after the EU referendum. While this was lower than the 0.7% it reported in the second quarter, it was better than the market forecast of 0.3% growth.

S&P said this GDP data was an important factor in determining the future rating. It said future GDP coming in less-than-expected could result in a further downgrade of the UK's credit rating, a barometer that would give investors an insight into the level of risk associated with investing in a particular country.

The American agency said there were also other factors that could lead to a downgrade. These included sharp falls in the UK currency and a new referendum on Scottish independence. It said the former could happen if foreign central banks decide to offload some of their sterling holdings, according to Reuters.

With regards to economic growth going forward, S&P said it expected the UK to grow by an average of 1% a year from next year to 2019. This is in line with the rating agency's June outlook.