The opening of the FTSE 100 and FTSE 250 indexes was delayed by what the exchange described as a "trading services issue."
Barclays (BCS), BP (BP), HSBC (HSBC) and Rio Tinto (RIO) are among the companies that make up the FTSE 100.
London's stock market previously suffered a technical glitch in June 2018 that delayed the start of trading by one hour.
If completed, the deal would combine the data generated by the exchange with Refinitiv's global distribution and analytics.
It would also bolster the UK company's position in foreign exchange and fixed income trading through Refinitiv's FXall and Tradeweb platforms.