The U.K. government faces an 80% chance of a credit rating downgrade if its deficit reduction plans remain as they are, Scott Mather, Pacific Investment Management Co.'s head of global portfolio management told Dow Jones Newswires Tuesday.

Mather also said yields on U.K. government bonds - known as gilts - could rise by as much as 100 basis points when the Bank of England's bond-buying program ends,

Asked if the U.K. faced a serious risk of suffering a downgrade to its credit rating, Mather said "I think so."

"It's just a question of when on the current trajectory, not if," Mather said. "Based on what we know today about the debt trajectory and about the inability to adjust that, I think it's greater than a 50% likelihood for sure. Call it more like 80%."

Mather said the government's debt reduction plan "is lacking in conviction and it is lacking in details."

He also said the end of the Bank of England's bond-buying program will have a significant impact on U.K. gilt markets and borrowing costs.

"Common sense would tell you that if you had a buyer in the market place which was taking the majority of the sector repeatedly... and then they disappeared, ...you would expect a reprising, and it could be quite significant," he said in a telephone interview.

"The estimates vary. They're really all over the map, but it could be 50 basis points, it could be 100 basis points, in that range."

PIMCO runs the Total Return fund--the world's biggest bond fund.

-By Laurence Norman, Dow Jones Newswires; 44-207-842-9270; laurence.norman@dowjones.com