The US has lost its last triple-A credit score from a major ratings firm after being downgraded by Moody's, which cited growing federal debt over the past decade.

In lowering the US rating to 'Aa1', Moody's noted that successive US administrations had failed to reverse ballooning deficits and interest costs.

A triple-A rating signifies a country's highest possible credit reliability, and indicates it is considered to be in very good financial health with a strong capacity to repay its debts.

Moody's warned in 2023 the US triple-A rating was at risk. Fitch Ratings downgraded the US in 2023 and S&P Global Ratings did so in 2011.

The downgrade "reflects the increase over more than a decade in government debt and interest payment ratios to levels that are significantly higher than similarly rated sovereigns," Moody's said in the statement.

A lower credit rating means countries are more likely to default on their sovereign debt, and generally face higher borrowing costs.

Moody's maintained that the US "retains exceptional credit strengths such as size, resilience and dynamism and the continued role of the US dollar as the global reserve currency".

The BBC has reached out to the US Department of Treasury for comment.