Brussels bomb suspect Najim Laachraoui is still on the run after the blasts which killed at least 31 people and left around 200 injured, including four Britons.
An arrest in connection with the terror attack has reportedly been made in the Anderlecht suburb of Brussels, but local media reports that Laachraoui had been detained were later withdrawn.
The 24-year-old, who has been linked to the atrocities in Paris five months ago, is believed to have accompanied brothers Brahim and Khalid El Bakraoui to Zaventem Airport on Tuesday, where they blew themselves up, killing 14 people.
An explosion an hour later on a train at Maelbeek Metro station killed a further 20 people and sent Brussels into lockdown.
Bomb maker Laachraoui is also suspected of having played a “decisive role” in the Paris terror attacks, French media has reported.
Laachraoui’s DNA was reportedly discovered on explosive belts found at the Bataclan theatre and the Stade de France following the killings in the French capital.
As it entered a second day of mourning, Belgium held a minute’s silence in memory of the victims.
The first confirmed fatality was mother-of-two Adelma Tapia Ruiz, 37, from Peru.
One Briton, David Dixon from Hartlepool, has been named as missing by his family who are said to be “desperately” searching for him.
Downing Street said it was concerned about a missing British national, adding that four Britons had been injured, three of whom were still in hospital.
Joining other world leaders in expressing their horror in the aftermath of the attacks, the Queen and Duke of Edinburgh said they were “deeply shocked and saddened”.
IS has claimed responsibility for the latest attacks and issued a statement in Arabic and French which threatens other countries in the anti-IS coalition with “dark days”, according to the SITE Intelligence Group, which monitors jihadi websites.