In addition, the ratings agency placed Barclays on negative watch, as fears mounted that new stress tests in Europe might leave the sector requiring extra capital to the tune of billions of pounds.
Lloyds and RBS both saw their levels downgraded to A from AA-minus, with Fitch noting: "Support dynamics are changing in the UK."
Such a move follows on from similar announcements recently made by fellow ratings company Moody's Investors Service.
Fitch also considered the recommendations of the Independent Commission on Banking (ICB) - a group that came into being in June 2010 and is currently chaired by Sir John Vickers - which suggested banks ring-fence retail operations from their riskier practices.
"There is also more advance political will to reduce the implicit support for the country's banks," the firm went on to point out.
By Asim Shah