The number of agency workers is set to reach one million by 2020 if current growth trends continue, according to Resolution Foundation research.
Describing agency workers as "the forgotten face" in the debate around insecure work, the think tank estimates a full-time agency worker gets £430 less than an employee in the same role.
The current number of 865,000 has grown by 30% since 2011, it said.
Last week, the government announced a review into modern working practices.
The Resolution Foundation, which campaigns on issues around low pay, said women accounted for 85% of the growth in temporary agency workers.
It did not explain why that was, but it will form part of an 18-month investigation into the subject.
The organisation says the workers are disadvantaged not only by lower pay, but also because they are not entitled to sick or parental leave pay and are more easily dismissed.
Half of all agency workers say they work on a permanent basis and three-quarters work full-time, according to the report.
And while there are good reasons for companies to use agency workers, the associated pay penalty was concerning, it said.
Other findings include:
- Agency workers are most likely to be in the health and social work sectors, followed by manufacturing and business activities (e.g. conference organisers, consultancy services, literary agents)
- Ethnic minorities are three times more likely to be agency workers than white workers
- Nearly one in five (18%) are in London
- Six in 10 are UK nationals (compared with more than eight in 10 of the overall workforce)
"While zero-hours contracts are often in the news, agency workers are the 'forgotten face' of the modern workforce, despite being just as prevalent across the labour market," said Lindsay Judge, senior policy analyst.
"This fast-growing group is not just made up of young people looking for temporary employment as some have suggested, but instead includes many older full-time, permanent workers.
"With the prospect of higher inflation squeezing living standards in the years ahead it is important that the discussion of the non-traditional parts of work in modern Britain consider the relatively lower pay that agency workers receive compared to identical employees in similar jobs," she added.
She also called for the government to examine the issue and introduce an official measure of agency workers.
A spokesman for the Department for Business, Energy and Industrial Strategy said: "Everyone deserves to be treated fairly at work regardless of the type of contract they are on...
"The Employment Agency Standards Inspectorate regulates the sector and investigates every valid complaint made against an employment business."