The agency on Wednesday evening placed the UK on what is known as a Rating Watch Negative (RWN), meaning that it is actively monitoring the UK's credit rating with an eye to lowering it in the near future.
"The RWN on the UK's ratings reflects primarily the heightened uncertainty over the outcome of the Brexit process, and an increased risk of a disruptive 'no-deal' Brexit, where the UK would leave the EU without a withdrawal agreement in place," Fitch said in a statement.
"Fitch believes that a 'no-deal' Brexit would lead to substantial disruption to UK economic and trade prospects, at least in the near term," it added.
Britain currently holds an AA rating from the agency, two notches below the highest possible AAA rating. It last held an AAA from Fitch in 2012, but saw a downgrade during the early years of David Cameron's coalition government, and another soon after the Brexit referendum.
The warning comes as the UK struggles to pass a Brexit deal through parliament, and the country's established political parties fracture. Parliament has until March 29 to pass a deal or risk the UK falling out of the EU without one, a scenario many analysts now believe is possible.
A ratings downgrade would not merely be a symbolic gesture from the agencies, but would also materially impact the UK's ability to access financing. The lower a country's credit rating is, the higher interest rates it must pay when accessing debt in the markets.
It can also become more difficult to find entities willing to provide funding, although it is worth remembering that even if downgraded a notch Britain's rating with both agencies would still be among the highest in the world.
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