Sky News has learnt that the Recapitalisation Group (RCG), a panel headed by the former Aviva chairman Sir Adrian Montague, will say this week that it has reduced its forecast for the overall level of unserviceable borrowing from more than £100bn to approximately £70bn.
The sharp decline will be based on the latest data from the Bank of England and Office for Budget Responsibility, and will be attributed to the UK economy rebounding more rapidly than expected from the initial coronavirus downturn.
Insiders said the RCG, which is sponsored by the lobbying group TheCityUK, would also significantly cut its £35bn estimate of the level of unsustainable debt created by the government's own emergency lending schemes during the crisis.
The group is expected to say that take-up of programmes including the Bounce Back Loans Scheme and the Coronavirus Business Interruption Loan Scheme have been lower than anticipated when figures were originally outlined in the spring.
Although in one sense the revised estimates will send a positive signal about the prospective level of unsustainable corporate borrowing, they will still highlight the risks of loan defaults worth tens of billions of pounds.
One source said that the default rate on Bounce Back Loans - which are capped at £50,000 per business and are entirely underwritten by the government - may be moderately higher than originally forecast.
The latest figures will come just weeks before the end of the Treasury's furlough scheme and, depending on the shape of the economic recovery in the coming months are likely to be subject to further adjustments.
In July, the RCG said the government should establish a 'UK Recovery Corporation' to help tackle the £35bn unsustainable state-backed debt burden accumulated by British businesses during the coronavirus crisis.
The Treasury has been lukewarm about creating a state agency to help recover the emergency loans, and has made it clear that it does not share the City grandees' pessimism about the scale of likely loan defaults.
TheCityUK declined to comment on Monday.