The global stock market rally represents a gamble by investors that central banks will ignore the risks of a buildup in debt and continue to provide support at the current record levels, the International Monetary Fund has warned .

In an update to its half-yearly global financial stability report, the IMF said central banks had been pivotal in the recovery of share prices from their Covid-19 trough but there was now a gap between the optimism of financial markets and the depressed state of economies.

The IMF said investors were “apparently betting on continued and unprecedented support by central banks”, adding that the disconnect between markets and the real economy raised the risk of another slump in asset prices that would harm recovery prospects.

“Markets appear to be expecting a quick ‘V-shaped’ rebound in activity”, the the report said, noting that the main bellwether of Wall Street – the S&P 500 – was out of kilter with signs of a deep downturn in the US.

“This has created a divergence between the pricing of risk in financial markets and economic prospects,” the IMF said.

The world’s major central banks have bought $6tn (£4.75tn) of assets since January, more than double the amount purchased during the global financial crisis of 2008. Unprecedented support meant share prices were back to 85% of their pre-crisis levels, but the IMF said there was no guarantee the upbeat mood would last.

Share prices have been under pressure in recent days, with the rising number of Covid-19 cases across the US leading to a more nervous mood.

The IMF said a second wave of infections was one of a number of triggers that could send markets lower. Such an outcome “could add financial stress on top of an already unprecedented economic recession”.

The report said expectations about the extent and length of central banks’ support to financial markets could prove too optimistic as trade tensions could resurface, and there was a risk of social unrest flaring up around the world in response to rising inequality.

It added that after a decade of low interest rates and money creation by central banks corporate and household debt were at high levels. The pandemic could bring these underlying vulnerabilities to the surface.

The IMF said there were “now many economies with high levels of debt that are expected to face an extremely sharp economic slowdown. This deterioration in economic fundamentals has already led to the highest pace of corporate bond defaults since the global financial crisis, and there is a risk of a broader impact on the solvency of companies and households”.

A surge in insolvencies would test the resilience of banks, even though they were in better shape than they were going into the crisis of 2008.

The IMF said central banks needed to be aware of the risk that short-term emergency action might have adverse long-term consequences.

“The unprecedented use of unconventional tools has undoubtedly cushioned the impact of the pandemic on the global economy and lessened the immediate danger faced by the global financial system. However, care needs to be taken to avoid a further buildup of vulnerabilities in an environment of easy financial conditions.

“Once the recovery is firmly under way, policymakers should urgently address financial fragilities that could sow the seeds of future problems and put growth at risk in the medium term.”