The department is looking to introduce more firms into the scheme - which is designed to send employee PAYE information as it happens - before it becomes compulsory next year.
It will represent the third expansion of the pilot, as HMRC looks to see where any issues can be smoothed out.
David Gauke, Exchequer Secretary to the Treasury, said that there will be cost savings available to businesses.
"RTI will improve the operation of PAYE and reduce administrative burdens on employers by £300 million every year," he explained.
"It will also provide up to date information about wages and tax for the forthcoming Universal Credit, so eligible employees and pensioners will get the right amount of benefits."
However, research by the Federation of Small Businesses, published last month, revealed that a quarter of small businesses are still unaware of RTI.
Posted by Emily Smith