The FTSE small cap index was subject to a trading halt during the outage, affecting some 222 stocks, including Tullow Oil, CMC Markets and Marston's.

LSEG said trading was being resumed.

"Instruments will go into auction at 09:55 with uncrossing beginning at 10:15. All live orders remain on the system," it said in a notice published on its website.

An LSEG spokesperson did not comment beyond the notice.

FTSE 100, FTSE 250 and International Order Book securities - shares listed in London by overseas companies - had continued to trade normally, the exchange said.

It is the second time LSEG has flagged a disruption to trading in smaller stocks on the London market in less than two months. On Oct. 19, an outage was caused by a technical problem, according to LSEG.

In 2019, the London Stock Exchange suffered an almost two-hour outage that hit FTSE 100 and midcap stocks, which LSEG said was caused by a "technical software issue".

Thomson Reuters, which owns Reuters News, has been a shareholder in LSEG since 2021. LSEG also pays Reuters for news stories.

(Reporting by Danilo Masoni and Alun John; Editing by Amanda Cooper, Kirsten Donovan)