More than $1tn (£760bn) has been wiped off the value of the cryptocurrency market in the past six weeks amid fears of a tech bubble and fading expectations for a US rate cut next month.

Tracking more than 18,500 coins, the value of the crypto market has fallen by a quarter since a high in early October, according to the data company CoinGecko.

Bitcoin has fallen by 27% over the same period to $91,212, its lowest level since April.

Investors around the world are on edge as fears mount over an artificial intelligence bubble in the stock market, with even the boss of Google’s parent company warning that “no company” will be immune if the bubble bursts.

The UK’s blue-chip FTSE 100 index was down by 1.2% on Tuesday, its fourth day in the red in a row. The Stoxx Europe 600, which tracks the biggest companies on the continent, fell 1.2%.

It follows steeper falls in Asia, where in the Japan the Nikkei 225 index shed 3.2%. Hong Kong’s Hang Seng index dropped 1.7%.

Sundar Pichai, the head of Google’s parent company, Alphabet, said in an interview with the BBC that there was “irrationality” in the current AI boom. He warned that in the event that the AI bubble bursts, “no company is going to be immune, including us”.

The chief executive of Klarna, Sebastian Siemiatkowski, also sounded the alarm this week, warning that huge sums being poured into computing infrastructure made him “nervous”.

He told the Financial Times: “I think [OpenAI] can be very successful as a company but at the same time I’m very nervous about the size of these investments in these datacentres. That’s the particular thing that I am concerned about.”

The Klarna co-founder added that the rising valuation of AI companies, including the chipmaker Nvidia, was also a source of concern. Nvidia became the first company to hit a market value of $4tn this year, later followed by Apple and Microsoft.

“That makes me nervous, because of the amount of wealth that is currently automatically allocated into this trend, without some more thoughtful thinking,” Siemiatkowski said.

“You can say, ‘I disagree with the fact that Nvidia is worth that much and I don’t care, some rich people are going to lose some money.’ But the truth is, because of the index funds and how this works, your pension right now is going into that theory that it is a good investment.”

An AI bubble is now seen as one of the most serious risks in the stock market, and a survey by the Bank of America found that 45% of its polled fund managers believe it is the biggest tail risk.

The price of gold, which is traditionally seen as a safe haven asset, is also falling. The spot price fell by 0.3% to $4,033.29 an ounce on Tuesday morning, after earlier hitting its lowest level in a week.

The drop comes amid fading expectations that the US Federal Reserve will cut interest rates next month. Higher interest rates make gold relatively less appealing as the metal does not pay a yield.

However, Giovanni Staunovo, an analyst at the Swiss investment bank UBS, said the gold price was likely to fall further but would soon recover.

“I would expect gold prices to bottom out soon, as I still see the Fed cutting rates several times over the coming quarters, and central banks’ diversification into gold remains strong,” he said.