“Negotiations are ongoing but subject to commercial, operational and regulatory discussions,” the financial services group said.
The discussions are taking place with online broker Interactive Investor, according to Sky News.
It is not certain whether the transaction will take place now or at all, Equiniti said.
Privately owned Interactive Investor, which has roughly £36bn in assets under administration, is looking to snap up the SIPP platform as it gears up for a London initial public offer, reports indicate.
II had agreed to shell out around £50mln for the Selftrade business, or EQi as it is currently branded, according to Sky.
Equiniti, where various parts of the business have been struggling in recent years due to the low interest-rate environment, sold off two of its business in December: Charter Systems, a provider of software for covert intelligence, to Northgate Public Services; and HR Solutions, its HR and payroll business, to Civica.
Interactive Investor has been contacted for comment.