Bank staff worked through the night on Tuesday and into Wednesday morning to prepare the unprecedented package, which will see it buying up a potentially limitless number of targeted government bonds.

It is the latest financial consequence of the market movements in recent days, which followed Kwasi Kwarteng's fiscal statement on Friday.

Since that statement, the pound has fallen sharply against most other major currencies and the value of UK government bonds has fallen, pushing up their implied interest rates or yields.

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That whipsaw reaction has had dramatic consequences for the so-called "liability-driven investors" sector upon which defined benefits pension schemes are reliant.

According to those with knowledge of the programme, the Bank's market operatives had begun to detect a "run dynamic" emerging from yesterday, with a number of pension funds suddenly not financially viable.

That in turn threatened the finances of many major banks, which would have faced severe troubles if those funds had collapsed.

According to those insiders, numerous funds were heading for collapse as soon as this afternoon.

Its dramatic action was designed to head off that outcome.