A few weeks ago, investors had begun to price in the chance that the BoE might raise interest rates this month for the first time in a decade. Thursday's decision sent yields on 10-year U.K. government debt tumbling to their lowest since June 28.

"It's pretty dovish testimony. They are again walking back policy," said Tom di Galoma, a managing director at Seaport Global Holdings in New York.

Weak US non-manufacturing data also boosted bonds.

The Institute for Supply Management (ISM) showed its non-manufacturing index fell to a reading of 53.9 last month from 57.4 in June. A reading above 50 in the ISM index indicates an expansion in the services sector, which accounts for more than two-thirds of the US economy.

Benchmark US 10-year notes gained 5/32 in price to yield 2.24 percent, down from 2.26 percent late on Wednesday.

The Treasury yield curve also continued to flatten, a day after the US Treasury Department said it was still considering an ultra long bond, but didn't announce a new issue.

The Treasury gave no timing for when it may make a decision on the bond. It also said it has begun to consider how it will increase debt issuance to make up for a future decline in US Federal Reserve bond purchases.

The yield curve between five-year notes and 30-year bonds flattened to 101 basis points, the lowest since July 25.

Large block trades in bond futures contracts may have helped boost the long-dated debt.

A block of 7453 contracts was purchased on Treasury bond futures at 6:37 EDT on Thursday, after a block of 7483 contracts was bought at 8:52 EDT on Wednesday, according to data by the CME Group.