The downgrade was the result of "significant budgetary underperformance in 2009", with the country's debt reaching $167bn, 76.6 per cent of gross domestic product.
Portugal's minority Socialist government responded by urging parliament to back a series of austerity measures aimed reducing the public deficit from 9.3 per cent of output to under the eurozone limit of 3.0 percent by 2013.
"Under the current situation of nervousness and volatility in the international financial markets ... it is essential for Portugal to show a firm political commitment to implement its growth and stability programme," the finance ministry said in a statement.
The programme, which forecasts slight economic growth of 0.7 per cent this year, includes a four-year freeze of government salaries, a reduction of social benefits and a delay in public investment.
The conservatives, who hold 102 of 230 seats, have yet to indicate how they will vote while 31 MPs from left-leaning parties will vote no. The Socialists hold 93 seats.
Fernando Teixeira dos Santos, the finance minister, warned that were the vote to fail "economic players would not believe in our ability to correct the deficit and control the debt, which would undermine confidence and would compromise the prospects for our economy."
The credit rating announcement came a day before eurozone leaders were expected to meet in Brussles to discuss a mechanism to allow debt-ridden Greece borrow more cheaply.
Greece faces the prospect of refinancing 20 billion euros of debt in April and May at twice the rate Germany has to pay to borrow.
Analysts cautioned that conditions in Portugal were still seen as more favourable than those in Greece.
"The market is taking it very well. AA- is still respectable credit and bears no comparison with Greece," Kenneth Broux, an economist at Lloyds TSB, said. Fitch rates Greece BBB+.
However, Fitch warned the outlook remains negative and that a further downgrade could be in the offing if the recovery is not as strong as anticipated in the next two years.