British government bond prices tumbled on Monday, in a sign that investors are yet to be convinced by Finance Minister Kwasi Kwarteng's drive to shore up fiscal credibility, which included bringing forward his new fiscal plan to October 31.

The plan had previously only been due on November 23 – two months after Kwarteng triggered a rout in British bonds by announcing a "mini-budget" that included 45 billion pounds ($50 billion) of tax cuts but no details on how they would be funded.

The finance ministry also announced that a civil servant with extensive budgetary experience, James Bowler, will head its staff. Kwarteng's sacking of his predecessor Tom Scholar had added to investor nerves around Britain's new economic agenda.

Despite the raft of announcements, yields shot up across the range of British debt maturities including in the long-dated conventional gilt market, which is subject to Bank of England (BOE) support measures that are due to expire on Friday.

The BOE expanded the scope of its emergency intervention on Monday.

"The market reaction so far has been far from encouraging and are a sign of how precarious the situation may still be," said Daniela Russell, head of UK rates strategy at HSBC.

The sharp rise in yields was specific to Britain, as French, German and Italian bonds showed much smaller increases in yields.

"The gilts market may be vulnerable this week as the end date for the BOE interventions nears," Rabobank analysts said.

J.P. Morgan strategists said they thought long-dated British yields would continue to move higher although it would be "a stretch" for the 30-year yield to retest the peak above 5 percent hit just before the BOE intervened in the market.

Even so, 20- and 30-year gilt yields rose were up more than 30 basis points (bps) on the day as of 1500 GMT, with the 20-year yield at one point reaching 4.945 percent.

Two-year yields leapt 35 bps to 4.42 percent.

In the index-linked market, yields were mostly up more than 60 bps on the day across all bonds with a maturity of 10 years or more. Many of these bonds have lost more than 50 percent of their value this year.

Linkers are less liquid than the conventional market and their ownership is dominated by pension funds which struggled to meet margin calls caused by the collapse in gilt prices following Kwarteng's September 23 mini-budget.

The gap between conventional 10-year British and German bond yields ballooned above 220 bps close to the 32-year high struck before the BOE intervened.

The BOE has said it is not targeting a specific yield, and bought 853 million pounds of conventional gilts with maturities of at least 20 years on Monday, while rejecting 263 million pounds. Total offers were far below the 10 billion pounds the BOE said it was open to buying.