London-based Aquis was founded by Alasdair Haynes, who was once touted as a potential CEO of the London Stock Exchange, whose junior market AIM it will list on.

The float would value Aquis at 73 million pounds.

Aquis accounts for 1.9 percent of European share trading in a sector led by the LSE and CBOE Europe, which represent 18 percent and 19 percent respectively, Aquis said.

Aquis has set itself a target of 3 percent of overall pan-European market share by the end of this year.

"There is a clear regulatory drive for greater transparency in trading and a requirement for market users to show they are using the best possible venue," Haynes said in a statement.

Haynes helped to build Chi-X, now CBOE Europe, into the biggest pan-European equities platform.

Aquis got the green light to trade shares in November 2013 and its "subscription-based" pricing was greeted with some scepticism at the time.

Stock exchange users are typically charged a percentage of the value of each stock trade, but on Aquis they pay a rate according to the level of activity, making it attractive to small trading firms in particular.

It also bans "aggressive" high frequency traders who dart in and out of the market in nanoseconds, providing a large slice of liquidity to big bourses but putting off some investors.

Trading in Aquis shares are due to start at 0700 GMT on June 14 with a placing price of 269 pence per share.

Aquis said it has conditionally raised 12 millions pounds, before expenses, with selling shareholders receiving 20.1 million pounds.

One of the selling shareholders, Warsaw bourse operator GPW, had agreed in 2013 to take a stake in Aquis for 5 million pounds. It announced on Monday it had sold its 20.3 percent stake for 12.4 million pounds gross.

Haynes is not selling any of his stake during the IPO. Aquis employs 31 people and made a pre-tax loss of 3.3 million pounds in the year to December 2017.

(Reporting by Huw Jones; editing by Jason Neely)

By Huw Jones