Fitch has puts the UK on a negative watch - not a downgrade.
It will be a relief for the Treasury at the end of a big week that it is only a watch and not the loss of AAA status yet.
But it does mean that a downgrade from a second agency is now more likely by the end of April. This could be followed by a third downgrade, from Standard and Poor's.
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Guillier conceded a "severe defeat" defeat to his supporters as the result came in.
The outage struck Hartfield-Jackson Atlanta International Airport at around 1pm local time, stopping all outgoing flights.
It will be a chilly day for many parts with the highest temperatures, about 10 Celsius (50F) in the far northwest.