Wall Street’s rebound since late March has been driven by the smallest number of stocks on record, pushing US market concentration to an all-time high and prompting warnings over the “fragility” of the rally. The S&P 500 index has soared more than 12 per cent since the start of April, when a ceasefire in the Middle East war sparked a storming rally, propelled largely by a handful of Big Tech stocks.

UBS said a measure of how many stocks were materially contributing to the performance, so-called “effective constituents”, hit a record low of 42 last week, far below the level of about 100 that has been typical in recent decades.

Before the conflict began, the equalweight version of the S&P powered ahead of the more commonly used index, in which bigger companies are weighted more heavily. But that pattern has reversed during the market rebound, with the S&P, which is dominated by Silicon Valley’s megacap tech companies, performing far better than its equal-weight counterpart.

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