The London Stock Exchange is attempting to renegotiate its deal to buy LCH.Clearnet, which plays a key role in clearing share trades, after regulators demanded it hold more cash.
The LSE agreed in April to buy 60 per cent of the company for €463m (£376m) and cleared a key hurdle yesterday after the Office of Fair Trading said the deal could go ahead.
But analysts said it could likely secure a lower price because of new European rules, which could leave the clearing business needing another €375m.
Clearing houses have to ensure trades are completed if a bank or another party goes bust.
That could leave them facing a huge bill and regulators have become increasingly concerned that they don't hold enough capital were another major bank to go to the wall.
LSE chief executive Xavier Rolet has been seeking to diversify the business.