The Bank of England has defended last week’s intervention in the UK government debt market, saying it stepped in to prevent a £50bn fire sale of gilts that would have taken Britain to the brink of a financial crisis.

The central bank said on Thursday that had it not launched its emergency bond-buying scheme in the wake of chancellor Kwasi Kwarteng’s “mini” Budget, pension funds would have been forced to sell £50bn worth of long-term UK government debt “in a short space of time”. This would far exceed the average daily trading volume of £12bn.

The BoE’s defence of the £65bn scheme is the clearest sign yet of how close the UK came to market meltdown following the Kwarteng’s “mini” Budget with £45bn in unfunded tax cuts.

Had the central bank not intervened, it feared there would have been a “self-reinforcing spiral” that threatened “severe disruption of core funding markets and consequent widespread financial instability”, said Sir Jon Cunliffe, the BoE’s deputy governor for financial stability, in a letter to the chair of parliament’s Treasury committee.

The letter also provided details on warnings received by the BoE ahead of its intervention. Cunliffe said that managers of the liability-driven investment strategies at the centre of a crisis in Britain’s pension fund industry had warned as early as Friday, September 23 that the huge moves in gilt yields would force them to dump large quantities of government debt.

The warnings continued during the day and into the evening on Monday, although a fall in 30-year gilt yields on Tuesday morning offered hope of “a somewhat more orderly liquidation of long-term gilts by LDI fund managers”, Cunliffe’s letter said.

But as the selling resumed, a number of LDI investors warned that their funds were likely to fall into net asset value, forcing them to wind up leveraged bets on interest rates that they use to manage the swings in the value of their long-term liabilities to pensioners. That in turn would have led banks that lent to the funds to sell the gilts used to back these bets, creating a downward spiral in gilt prices.

Long-term borrowing costs, which had surged to their highest level in two decades, plunged after the BoE announced its intervention. The overall move in 30-year gilt yields on September 28 of 1.27 percentage points was larger than the annual trading range for the bonds in all but four of the last 27 years, the letter said.

The BoE has spent just £3.7bn in the first six days of its programme, far below the potential maximum of £30bn. Investors and traders in the gilt market say the possibility of larger central bank purchases has been successful in backstopping the market and giving other buyers the confidence to step in.

However, yields have begun to move higher again this week after the BoE bought no bonds at all on Tuesday and Wednesday. Thirty-year borrowing costs traded at 4.37 per cent on Thursday morning, up from a low of 3.64 per cent in the wake of the BoE’s intervention but below the 20-year high of more than 5 per cent that forced the central bank to act.

Cunliffe also responded to concerns that the gilt-buying intervention, which is due to end on October 14, undermines the BoE’s fight to bring inflation under control by in effect loosening monetary policy. “These operations are not intended to create central bank money on a lasting basis, nor are they designed to cap or control long-term interest rates,” he wrote.

Alongside last week’s gilt purchases, the central bank also delayed the start of sales of gilts from its portfolio in a bid to shrink its balance sheet — a process that forms part of the BoE’s plans to tighten monetary policy. Cunliffe reiterated that this so-called “quantitative tightening” is scheduled to begin on October 31.