There are “strong” links between June’s Tunisia beach massacre and the killings at the Bardo Museum in the country’s capital in March, British police say.
Metropolitan Police counter-terrorism officers say they are now connecting the two attacks based on evidence.
The Bardo Museum attack saw 22 people killed, while 38 tourists, including 30 British nationals, were killed in the resort of Sousse in June.
Tunisian authorities have arrested 150 people to date over the Sousse attack.
Of those, 15 have been charged with terrorism offences.
Commander Richard Walton, of the Metropolitan Police, which supplied officers to help the Tunisian investigation into the massacre, said gave no details of the suspected connection between the attacks but said it was based on “strong” evidence.
In Sousse a gunman, who was later identified as Tunisian student Seifeddine Rezgui, opened fire on the beach after coming in from the sea using a jet ski or speedboat.
After shooting on the beach, he entered the Hotel Imperial Marhaba, where explosives were detonated and more tourists were shot. Rezgui then ran out of the hotel and police shot him dead.
Tunisian authorities believe the 23-year-old’s suspected accomplices provided a Kalashnikov assault rifle and helped Rezgui get to the scene, interior ministry spokesman Mohamed Ali Aroui told AP.
The militant group Islamic State (IS) later said it was behind the attack.
In Tunis – Rana Jawad, BBC North Africa correspondent
The announcement by the Metropolitan Police’s counter-terrorism unit is not surprising, but it provides some credence to what has so far remained cryptic rhetoric from Tunisian officials.
Less than a week after Saifeddine Rezqui’s rampage in Sousse, authorities here said the gunman had “likely” trained in Libya at the same time as the Bardo Museum assailants.
How and why they reached that conclusion was unclear.
By mid-July security services here had arrested more than 100 people they describe as being “suspected members of terrorist gangs” – in the majority of these cases, it’s unknown what happens to these detainees or what “gangs” they belong to.
The Bardo Museum in Tunis was stormed by three gunmen on 18 March. British, Japanese, French, Italian and Colombian tourists were among the 22 they killed.
Witnesses said the gunmen, carrying assault rifles, opened fire on tourists outside the museum in front of a row of buses before charging inside and taking hostages.
Two of the gunmen – identified by the authorities as Yassine Laabidi and Hatem Khachnaoui – were killed by security forces.
In an audio message posted on Twitter accounts known to be sources of IS propaganda, the militant group claimed responsibility for the attack and praised the dead men as “knights of the caliphate”.
Nine people have now been arrested in connection with the museum attack.