For the most part, when people think about the Bank of England and what it does to control the economy, they think about interest rates.

And that’s quite understandable. After all, influencing inflation by raising or lowering the prevailing borrowing costs across the UK has been the Bank’s main tool for the vast majority of its history. There are data series on interest rates in the Bank’s archives that go all the way back to its foundation in 1694.

But depicting the Bank of England as being mostly about interest rates is no longer entirely true. For one thing, these days it is also in charge of regulating the financial system. And, even more relevant for the wider economy, it is engaged in another policy with enormous consequences – both for the markets and for the public purse. But since this policy is pretty complex, few outside of the financial world are even aware of it.

Money latest: What interest rate hold means for you

That project is quantitative easing (QE) or, as it’s better known these days, quantitative tightening (QT).

You might recall QE from the financial crisis. It was, in short, what the Bank did when interest rates went down to zero and it needed an extra tool to inject some oomph into the economy.

That tool was QE. Essentially it involved creating money (printing it electronically) to buy up assets. The idea was twofold: first, it means you have more money sloshing around the economy – an important concept given the Great Depression of the 1930s had been associated with a sudden shortage of money. Second, it was designed to try to bring down the interest rates prevailing in financial markets – in other words, not the interest rate set by the Bank of England but the yields on long-dated bonds like the ones issued by the government.

So the Bank printed a lot of money – hundreds of billions of pounds – and bought hundreds of billions worth of assets. It could theoretically have spent that money on anything: stocks, shares, debt, housing. I calculated a few years ago that with the sums it forked out, it could theoretically have bought every home in Scotland.

But the assets it chose to buy were not Scottish homes but government bonds, mostly, it said back at the time (this was 2009) because they were the most available liquid asset out there. That had a couple of profound consequences. The first was that from the very beginning QE was a technical policy most people didn’t entirely understand. It was all happening under the radar in financial markets. No one, save for the banks and funds selling government bonds (gilts, as they’re known) ever saw the money. The second consequence is that we’re starting to reckon with today.

Roll on a decade-and-a-half and the Bank of England had about £895bn worth of bonds sitting on its balance sheet, bought during the various spurts of QE – a couple of spurts during the financial crisis, another in the wake of the EU referendum and more during COVID. Some of those bonds were bought at low prices but, especially during the pandemic, they were bought for far higher prices (or, since the yield on these bonds moves in opposite directions to the price, at lower yields).

Then, three years ago, the Bank began to reverse QE. That meant selling off those bonds. And while it bought many of those bonds at high prices, it has been selling them at low prices. In some cases it has been losing astounding amounts on each sale.

Take the 2061 gilt. It bought a slug of them for £101 a go, and has sold them for £28 a piece. Hence realising a staggering 73% loss.

Tot it all up and you’re talking about losses, as a result of the reversal of QE, of many billions of pounds. At this point it’s worth calibrating your sense of these big numbers. Broadly speaking, £10bn is a lot of money – equivalent to around an extra penny on income tax. The fiscal “black hole” Rachel Reeves is facing at the forthcoming budget is, depending on who you ask, maybe £20bn.

Well, the total losses expected on the Bank of England’s Quantitative Tightening programme (“tightening” because it’s the opposite of easing) is a whopping £134bn, according to the Office for Budget Responsibility.

Now it’s worth saying first off that, as things stand at least, not all of those losses have been crystallised. But over time it is expected to lose what are, to put it lightly, staggering sums. And they are sums that are being, and will be paid, by British taxpayers in the coming years and decades.

Now, if you’re the Bank of England, you argue that the cost was justifiable given the scale of economic emergency faced in 2008 and onwards. Looking at it purely in terms of fiscal losses is to miss the point, they say, because the alternative was that the Bank didn’t intervene and the UK economy would have faced hideous levels of recession and unemployment in those periods.

However, there’s another, more subtle, critique, voiced recently by economists like Christopher Mahon at Columbia Threadneedle Investments, which is that the Bank has been imprudent in its strategy of selling off these assets. They could, he argues, have sold off these bonds less quickly. They could, for that matter, have been more careful when buying assets not to invest too wholeheartedly in a single class of asset (in this case government bonds) that might be sensitive in future to changes in interest rates.

Most obviously, there are other central banks – most notably the Federal Reserve and European Central Bank – that have refrained from actively selling the bonds in their QE portfolios. And, coincidentally or not, these other central banks have incurred far smaller losses than the Bank of England. Or at least it looks like they have – trying to calculate these things is fiendishly hard.

But there’s another consequence to all of this as well. Because if you’re selling off a load of long-dated government bonds then, all else equal, that would have the tendency to push up the yields on those bonds. And this brings us back to the big issue so many people are fixated with right now: really high gilt yields. And it so happens that the very moment Britain’s long-term gilt yields began to lurch higher than most other central banks was the moment the Bank embarked on quantitative tightening.

But (the plot thickens) that moment was also the precise moment Liz Truss’s mini-budget took place. In other words, it’s very hard to unpick precisely how much of the divergence in British borrowing costs in recent years was down to Liz Truss and how much was down to the Bank of England.

Either way, perhaps by now you see the issue. This incredibly technical and esoteric economic policy might just have had enormous consequences. All of which brings us to the Bank’s decision today. By reducing the rate at which it’s selling those bonds into the market and – equally importantly – reducing the proportion of long-dated (eg 30 year or so) bonds it’s selling, the Bank seems to be tacitly acknowledging (without actually quite acknowledging it formally) that the plan wasn’t working – and it needs to change track.

However, the extent of the change is smaller than many would have hoped for. So questions about whether the Bank’s QT strategy was an expensive mistake are likely to get louder in the coming months.