Trading on the London Stock Exchange in small cap stocks has been down for much of Tuesday morning with LSEG saying it was still investigating the technical issue after midday.
Initially, the company said just before 9.30am that only FTSE 100, FTSE 250 and IOB (international order book) stocks were available for trading.
A similar problem hit trading on the LSE in October when trading in all AIM stocks was completely suspended at one point.
Following this morning's initial outage, an update in a later statement informed that full trading would resume by 10:15am but an hour later the problems resumed, with just FTSE 350 and IOB shares trading.
"We are still investigating the issue," LSEG said in three successive statements until updates at 12.17pm and 12.43pm said: "Impacted instruments are being resumed" and then "Impacted securities are now in regular trading".
Share status was for a time shown as "halted" but LSEG said customers "will be able to manage their orders in the system" amid uncrossing of trades, while SETSqx securities set for uncrossing at 2pm, "as per the normal schedule after which the instrument status will move from Halted to a instrument status that reflects its regular schedule".
Pure quote driven securities will continue to show as Halted although quotes can still be managed in the system with investigations ongoing.
In the initial statement just after 9:23am, the company said there was an "issue impacting its trading/information system".