Rating agency Standard & Poor's has downgraded debt-stressed Portugal's credit worthiness, deepening the country's financial plight as it fights to avoid a bailout.

The agency said Tuesday it lowered its sovereign credit ratings on Portugal to BBB-/A-3 from BBB/A-2 and said the country's outlook is "negative."

It said Portugal's high debt load and poor growth prospects make it likely the ailing country will need a financial rescue package.

Analysts estimate Portugal would need a bailout of around 70 billion euros-80 billion euros ($98 billion-113 billion).

Portugal's government quit last week in a dispute with its rivals over a debt reduction plan — a development which renewed investor uncertainty and triggered a rise in its borrowing costs.