The firms announced this week they had reached agreement on the deal.
The ii group currently has over £30bn in assets under administration, over 300,000 customers and more than 1m users. The Share Centre has 271,000 customers with £4.9bn in assets under administration.
ii plans to acquire the entire issued share capital of Share plc, subject to shareholder, regulatory and legal approvals.
ii says the move has been partly driven by its view that to be profitable in the future consolidation in the sector among “compatible” groups is necessary.
ii and Share plc (parent of The Share Centre) say that a combination of the two businesses will create a group of “greater scale and capability to grow and thrive.”
Richard Wilson, chief executive of ii, said: “We are delighted with this transaction. The firms’ shared values and combined strengths reinforce ii’s position as a leader in the retail investment services marketplace. With our fair flat fees we have built a strong and compelling alternative to percentage fees, in a business that puts the customer first.
“Combining our individual strengths brings further scale and the opportunity to deliver enhanced value, service and customer experience to an enlarged customer base.”
Gavin Oldham, executive chairman of Share plc, said: "I am delighted to welcome this combination of our businesses, designed to transform the prospects for individual share ownership and personal investment across the United Kingdom.
“Our shareowners, employees and customers are well aware of my passion for egalitarian capitalism, not only right across society but also across generations. It delivers the economic freedom that comes from having a personal reserve of savings and investment, and a society at ease with itself: as owners, employees and consumers combine ownership with a responsibility for all.”
ii group has been running for more than 20 years and claims to be the UK's second largest direct to consumer investment platform.
Following the deal, The Share Centre’s clients will be moved to the ii platform.