“We conducted a sea trial and then returned to port to allow the storm to pass. This meant delaying our departure to avoid risking complications with the smaller boats,” the Global Sumud Flotilla Mission said in a statement, adding winds had been up to around 35 mph (56 kph).
The organizers did not say when they planned to resume the journey.
Activists including Game of Thrones actor Liam Cunningham are on the flotilla of several dozen boats that is aiming to break Israel’s naval blockade and deliver food and humanitarian supplies to the enclave, which has been devastated by nearly two years of war.
Red Cross head says mass evacuation of Gaza City ‘impossible’
Israel has argued that the naval blockade it imposed in 2007 is necessary to stop weapons being smuggled to the Hamas Palestinian militant group.
It has described other attempts to break it - including one that included Thunberg in June - as a propaganda stunt in support of Hamas.
A global fleet of boats, part of an international maritime initiative aimed at delivering humanitarian aid to starving people in Gaza, was forced to turn back after setting sail on Sunday due to bad weather.
In a statement, the organisers said that due to unsafe weather conditions, including strong winds exceeding 30 knots (55.5km/h), they decided to return to port to prioritise the safety of everyone on board.
The team has scheduled a meeting for Monday to decide when to resume their journey to the Gaza Strip, with the possibility of setting sail later in the day.
The first convoy, consisting of dozens of small civilian vessels carrying activists, humanitarians, doctors, seafarers, and humanitarian supplies, departed from Spanish ports on August 31 to meet up in Tunisia with a second wave on September 4.
Organisers describe the Global Sumud Flotilla as the largest maritime mission to Gaza, bringing together more than 50 ships and delegations from at least 44 countries.
According to the Global Sumud Flotilla, delegations from 44 countries have already committed to sail to Gaza as part of the largest maritime mission to break Israel’s illegal siege.
Countries from six continents will be taking part in the flotilla, including countries such as Australia, Brazil, South Africa and numerous European states
According to the group, participants are unaffiliated with any government or political party.