Fred Neumann, the chief Asia economist at HSBC in Hong Kong, said: “Rising debt levels in developed markets have become a growing concern in recent years, and the seeming lack of urgency about fiscal consolidation is leaving government bond investors increasingly jittery.

“After years of generous spending and tax cuts, fiscal constraints are starting to bite. Japan is not alone in facing the need for budget consolidation, but volatility in other developed economies’ bond markets adds an extra layer of urgency to put the fiscal house in order.”

British and Japanese bond yields are related because the two are substitutes for each other, according to Paul Dales, chief UK economist at Capital Economics. But he says the big risk to the UK comes from the US.

“The UK bond market is regularly buffeted by events in the US,” he said. “Uncertainty and tighter monetary policy in the US can be imported into the UK.”

The US, which suffered a downgrade from credit agency Moody’s last week, has a government debt-to-GDP ratio of 123pc and is rated by BNP Paribas as the most fiscally vulnerable developed economy.

An American “Liz Truss moment” is “a non-trivial possibility in current conditions”, the bank said on Tuesday. BNP Paribas expects Congress to tap the brake on the US deficit, but “perception can be reality in markets”.

Avoiding the tripwires

The ray of hope in Japan is that the markets will calm down and volatility will ease, even if yields never return to the level of previous decades.

Some investors, such as Vanguard and RBC BlueBay Asset Management, are doing some contrarian buying of 30-year Japanese bonds according to Bloomberg.

But that might not be enough to ease the squeeze, unless big Japanese money also starts to crowd in.

The next test of the Japanese litmus paper is barely a week away, when the government will hold an auction of 40-year bonds. Investors are already nervous: the 40-year yield was up more than 10 basis points on Tuesday.

Will Japan blow up the world economy, including Britain’s public finances?

There are so many tripwires: the risk appetite of Japanese investment managers; the deliberations of the Bank of Japan; the election promises of Japanese political parties; and the continuing emanations on tariffs and tax cuts from the White House and Capitol Hill.

And those are only the biggest threads.

It’s a hard knot to unpick. Reeves will just have to soak up BNP Paribas’s pithy observation: “In the short term, concerns about debt sustainability provide just one more reason to predict unpredictability.”