Bosses at whistleblower support organisations told Sky News that many workers are scared to report employers who are asking them to continue to work despite claiming furlough support.
New statistics from The Institute for Fiscal Studies revealed that a quarter of the nation's workforce - around 7.5 million people - are now covered by the scheme, which has cost £14bn a month.
Until the end of July, there will be no changes to the scheme but some changes will be phased in from August.
He said: "Supporting a part-time return to work from August is a sensible move which gives employers time to ensure workplaces are safe".
After July, employers will be able to bring furloughed staff back to work part-time.
In its official advice, the Treasury said: "The employer payments will substitute the contribution the government is now making, ensuring that staff continue to receive 80% of their salary, up to £2,500 a month".
Lex Butler, chair of the Hotel Booking Agents Association, has been one of the most vocal campaigners for the extension of the scheme.
Full details regarding the changes will be available by the end of May, but Sunak assured furloughed workers they will continue receiving the same level of overall support. The industry would have been decimated.
Sunak has previously said he was preparing to "wean" workers and businesses off the programme - which now runs until the end of June - but calls have been made for it to be prolonged.
Nicky Jolley, managing director of HR2day in Darlington, has warned businesses of "furlough fever" when trying to get employees back to work, potentially leaving them short-staffed.
"Perhaps the weaning off will not only be tapered as to contribution amounts but it may also only be maintained for those businesses where it is most needed", added Brown. Our message today is simple.
Sunak cited data saying that the job retention scheme has protected 7.5 million workers and almost one million businesses across the country.
The Road Haulage Association (RHA) said the furlough extension means firms will be better prepared to service inconsistent and unpredictable workloads as the economy begins its recovery.
"We will ask employers to start sharing, with the government, the costs of paying people's salaries".
- Can we expect to see furlough extended beyond October?
"Trading conditions will differ depending on location and sector and the flexibility of the furlough scheme must take these factors on board".
However the maintenance of a scheme costing £14 billion a month, more than the normal cost of the NHS is a price that will need repaying, with a leaked document today saying the Treasury is considering future tax increases, hardly a surprise there then, where else would the cash come from?