An intensifying global bond sell-off is piling pressure on UK public finances just ahead of a crucial budget announcement, with the latest debt sale revealing funding costs that have not been seen in nearly three decades.

Investor details from Tuesday's auction show that the government raised £4.25 billion through the sale of 30-year gilts at a yield of approximately 5.83%. This marks the highest interest rate ever recorded for a gilt auction since the UK Debt Management Office was established in 1998.

The elevated borrowing expense comes amid a confluence of factors, including the Iran conflict, concerns over excessive global debt supply, and a significant upward shift in yields since the pandemic. This auction has firmly confirmed that long-term government funding costs have reached a three-decade high.

Gordon Shannon, a portfolio manager at Twenty Four Asset Management, noted that while the current gilt market is entirely driven by global forces, the domestic cost of borrowing has still climbed substantially. He added that this will inevitably constrain the government's room for policy manoeuvre in next month's budget.

The record yield on new issuance highlights the mounting strain that rising interest payments are placing on public finances. The UK's annual interest bill has already reached £110 billion, an amount surpassing several other major government expenditures.

During a speech on Monday, Chancellor John Healey stated that if debt interest were treated as a government department, it would be the second largest in Whitehall, behind only health, and larger than the combined spending on defence, the Home Office, and the justice system. He also pledged to be candid about the necessity of controlling fiscal spending in the budget.

Investors indicated that UK gilt yields remain attractive compared with other countries, and the auction received solid demand. As Stephen Jones, Chief Investment Officer at Aegon Asset Management, part of Allianz, put it: "What is a problem for some is an opportunity for others. Investors see the yields currently on offer as sufficiently attractive and are choosing to add to gilt positions."

Orders for Tuesday's syndicated sale totalled over £85 billion. The Debt Management Office intends to issue £250 billion in gilts this year to finance government spending plans.

The more closely watched 10-year borrowing yield has reached 5.2%, the highest among G7 nations. Earlier this year, buoyed by rising yields, the Debt Management Office recorded its most expensive 10-year issuance since 2008.

Energy prices have surged since the outbreak of the Iran conflict, triggering a global bond sell-off and upending investor expectations for interest-rate cuts from major central banks, including the Bank of England. With the conflict still escalating, the global benchmark Brent crude price is nearing $100 per barrel again, adding further inflationary pressure and posing a growth shock to the global economy.

Long-term funding costs in other parts of Europe also hit new highs on Tuesday. France's 30-year bond yield climbed to 5.02%, its highest level since September 2008, while Germany's 30-year yield rose to 3.86%, the highest since 2011.

Market pricing suggests traders anticipate at least one 25-basis-point rate hike from the Bank of England before the end of the year. While a move at this month's policy meeting is widely considered unlikely, investors expect the central bank to slow the pace of its gilt sales, a process that has previously contributed to upward pressure on long-end yields.