Contractors, and businesses or agencies that engage them, need greater clarity before IR35 rules extend to the private sector next spring.
From April 2020, the off-payroll rules which already apply to contractors engaged in the public sector will be extended to the private sector.
Private-sector businesses or agencies that hire contractors will be responsible for determining whether an engagement falls within the rules from 2020/21.
These responsibilities, which are currently held by contractors themselves via their own personal service companies, include applying the correct tax and national insurance.
The Association of Tax Technicians (ATT) said HMRC has not provided details as to when liability could be transferred, such as in a labour supply chain.
Michael Steed, co-chair of ATT's technical steering group, said:
"Uncertainty in how the off-payroll rules will operate in practice is making it difficult for businesses to make adequate preparations.
"With about six months to get ready, much work remains to ensure private-sector engagers and agencies are both aware of the changes and ready for them.
"We encourage HMRC to release more information and detailed guidance as soon as possible."
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