Ford is trying to transform its US business

Fitch Ratings said it had carried out the move because it expects Ford's revenues will continue to deteriorate throughout 2006.

In April Ford announced a $1.19bn (�629m) quarterly loss as it struggles against weak sales in the US.

A poor credit rating makes it harder for a company to borrow money.

It indicates that a firm is more likely to default on a loan, meaning the company in question faces much higher interest rates.

'Deteriorating product range'

Fitch Ratings said Ford's revenues were continuing to struggle "due to continued market share losses, deteriorating [product] mix, price competition, a lack of key product introductions, coupled with lack of tangible progress in reducing its cost structure".

Ford is cutting 30,000 jobs and closing 14 plants by 2012 to reduce its costs.

While analysts have criticised Ford's vehicle range in the US, the firm's sales have also been hit by higher fuel costs.

To help temper US motorists' concerns about soaring petrol prices, Ford announced earlier this month that it is to give away at least $1,000 (�540) of fuel with new vehicles.

Rival General Motors has also started a similar scheme.