Clearing re-emerged as a battleground immediately after the Brexit vote, with French and German leaders arguing that euro operations shouldn't remain in London once the UK leaves. Regulations since the 2008 crisis boosted the importance of clearinghouses, requiring many derivatives transactions to pass through them.

UK clearing firms are able to serve EU customers for now because they're part of the bloc, but they will have to secure authorisation known as equivalence after Britain leaves. It took about four years of painful negotiations for the US and Europe to come to an agreement on clearing equivalence.

The over-the-counter derivatives market is massive, and London is a global hub for those transactions. LSE is the majority owner of LCH, which dominates clearing of interest-rate swaps, a market where some $US2.7 trillion of derivatives changes hands every day. LCH handles more than 90 per cent of cleared interest-rate swaps trades in major currencies. Its swap-clearing unit has more than 100 members.

Separately, CME Group said it was examining options in Dublin to ensure its clearinghouse keeps access to European Union customers after the UK leaves the bloc, people familiar with discussions say.

Managers at the Chicago-based derivatives exchange are weighing stronger ties to Ireland to ensure its London clearing operations aren't disrupted, but no decisions have been made, said the people, who asked not to be named because the conversations were private. CME's options in Dublin could include seeking out regulatory licensing or opening offices. A spokesman for CME declined to comment.

UK voters' decision to leave the EU is showing signs of undermining London's role in financial markets, with New York and Frankfurt frequently named as potential beneficiaries. CME's deliberations show Dublin is a contender, too.

The decision on where to base a clearinghouse, which play a vital role in derivatives by acting as a firewall from failed trading firms, is important because it could shift jobs and the balance of financial power from one city to another.

CME started its London-based European clearinghouse in 2011. While its clearing presence is still very small, the internal talks show how executives that previously relied on their London address are jockeying to protect their businesses. CME's European clearing business depends on its London location for EU access.

Although they don't run derivatives clearing, other UK companies in the exchange industry -- such as currency venue LMAX Exchange and Bats Global Markets's London unit -- are considering Dublin as a way to keep Brexit from disrupting their businesses. Ireland has been part of the EU for decades and uses the common European currency.

Bloomberg