Chancellor Rishi Sunak is being urged to extend the self-employed income support scheme (SEISS), which ends this weekend.
The scheme pays lump sums worth 80% of total profits, up to £2,500 a month, over a three-month period starting from March 2020.
More than two million of the UK's five million self-employed population had applied for the grants in the wake of the coronavirus crisis.
Payments are capped at £7,500 per worker, while the Treasury has stumped up more than £6.8 billion to successful applicants so far.
A similar scheme for furloughed employees has been extended until the end of October.
The Association of Independent Professionals and the Self-Employed (IPSE) wrote to the Chancellor, calling on him to extend the support scheme.
It warns that many firms are "fearing a financial cliff-edge" with the scheme due to close on Sunday 31 May 2020.
"We recognise the scheme cannot run indefinitely," said the IPSE in conjunction with three other bodies representing the self-employed.
"Given the employee job retention scheme will now run until 31 October 2020, a similar principle should be adopted for the SEISS - it should also be extended."
Talk to us about coronavirus-support schemes.