GLASGOW-based Clydesdale Bank's credit rating is under threat after Fitch Ratings warned it is considering downgrading parent company National Australia Bank (NAB) and three other leading Australian institutions.

Debt issued by Clydesdale, headed by David Thorburn, is currently rated at A+ by Fitch after being downgraded from AA- on September 30. But it faces another downgrade after being put on "ratings watch negative" as the agency warned of the Australian banks' funding challenges.

Australian banks have some of the strongest credit ratings in the world after avoiding many of the toxic assets that brought down banks in the UK and US.

Australia also avoided recession in the wake of the global financial crisis.

But Fitch put four banks – NAB, Commonwealth Bank of Australia, Westpac, and Australia and New Zealand Banking Group – on "ratings watch negative" because their funding profile is weaker than rivals and they face an uncertain economic backdrop and changing regulation.

Fitch said: "The agency views the Australian major banks' ratings as under pressure at their current levels. Despite significant improvements, these banks continue to have a weaker funding profile than other similarly rated peers."

Fitch said downgrades are likely to be limited to one notch and that banks currently rated at "AA", including NAB, are most likely to see a downgrade.

A downgrade could make it more expensive for the banks to raise funding.

A Clydesdale spokesman said: "Our long-term rating has not changed and is only under review. We are a conservative bank with sound capital and our parent, NAB, is a highly rated and well capitalised bank which continues to strengthen its profile."

Despite speculation that NAB would prefer to sell its UK business to focus on fast-growing Asia, last week it announced it is pumping £130 million into the Clydesdale pension scheme.