Research by the Caminando Fronteras (Walking Borders) charity revealed at least 6,618 people had died or disappeared while making the journey to Spain throughout last year. Record numbers headed for the Canary Islands.
The number is almost three times that of the previous year – 2,390 – and the highest since the charity began keeping a tally in 2007, co-ordinator Helena Maleno said.
Ms Maleno blamed the rise in migrant deaths and disappearances on a lack of resources for rescuers.
Nearly half involved migrants who had departed for Spain from Senegal, where political unrest along with a lack of jobs, rising food prices and dwindling fish stocks have caused thousands to flee the country.
The majority of deaths – 6,007 – took place on the Atlantic migration route from Africa to Spain's Canary Islands.
As controls are tightened in the Mediterranean, the seven islands have become a favourite destination for people fleeing poverty and conflict in Africa, mostly on overcrowded, barely seaworthy vessels that lack enough food and water for the journey.
“The Atlantic route has become the deadliest in the world,” Ms Maleno said.
The number of migrants arriving illegally in Spain in 2023 nearly doubled from the previous year, reaching 56,852, according to Interior Ministry figures.
That was the highest number since 2018, when 64,298 migrants entered the country.
The majority, about 70 per cent, arrived in the Canary Islands, which, at their closest point, are barely 100km from the north-west coast of Africa.
Boats depart from Morocco as well as Mauritania, Gambia and Senegal further south.
The 1,600km journey from Senegal to the Canaries usually takes a week of difficult upwind sailing.
Spain's central government in October pledged an aid package worth €50 million ($54 million) to help with the surge in arrivals.
Madrid has also stepped up co-operation with Senegal and Mauritania to try to stop boats leaving for the Canaries.
Spanish Interior Minister Fernando Grande-Marlaska said on Thursday this co-operation with African nations had stopped the departure of more than 27,000 would-be migrants to Spain last year.
“We have saved lives,” he claimed.
The International Organisation for Migration, which bases its tally on press reports and indirect accounts, said more than 1,200 migrants died or disappeared last year while trying to reach Spain.
But the UN agency stresses its figure is most likely “considerably” underestimated, given the difficulty of documenting shipwrecks and the fact that most bodies are never found.