The share registrar floated on the London Stock Exchange in October 2015 and Advent is in the process of selling its remaining eight per cent stake through an accelerated bookbuild.
The stake represents 23.6m shares and if sold at today’s closing price, 193.34p, would raise just short of £46m.
The price, which could be discounted, is expected to be disclosed tomorrow morning in a stock market announcement, the Evening Standard first reported.
Advent bought Equiniti, which handles most of the FTSE 100’s dividend payments, in 2007.
When the company floated last year, chief executive Guy Wakeley said:
The IPO will enable Equiniti’s management team to continue its proven growth strategy, while benefiting from a growing market and favourable trends towards outsourcing, greater regulation and digitalisation.
Advent and Equiniti declined to comment.