The two stock exchange operators announced the terms of what they called a "merger of equals" in a joint declaration on Wednesday.

A new holding company called UKTopCo will be created, based in London. But the merged company will maintain its operational headquarters in both Frankfurt and London.

"We're strengthening the connection between the two leading financial centers in Europe - Frankfurt and London - to build a European network with Luxembourg, Paris and Milan. That way we can strengthen capital markets in the whole of Europe," Deutsche Börse CEO Carsten Kengeter said in the declaration.

Kengeter will also serve as chief executive of the combined group, the LSE's Donald Brydon is to become chairman.

The two operators call the tie-up an "industry-defining combination," with their boards "committed to maintaining their [the two exchanges'] respective strengths and capabilities."

The London Stock Exchange stresses the merger would reinforce London's position as a leading financial center with its "outstanding talent pool." Kengeter emphasized the merger would go ahead even if the UK decided to leave the EU - also known as Brexit.

The new company would achieve cost synergies of 450 million euros ($500 million) annually after three years - equivalent to 20 percent of the combined group's 2015 operating costs, the declaration states.

The merger is still subject to shareholder and regulatory approval. It is the third attempt of the two operators to merge. Two previous attempts in 2000 and 2004 failed due to resistance by the LSE's owners and some big shareholders.

The two bourses had announced the latest merger talks at the end of February.