LONDON — The sea journey between North Africa and Italy is now deadlier than at any point since the peak of the European migration crisis in 2015, even as unauthorized migration along the route has fallen to its lowest level in the same period, according to data released on Monday by the United Nations.

For every 18 migrants who reached Italy by boat during the first seven months of 2018, one person drowned attempting that voyage. The toll is nearly triple the death rate during the period in 2015, and roughly double the rate in 2016, when a record number of migrants landed without authorization in Italy, which for most of the 21st century has been the main port of entry for people hoping to reach Europe by boat.

The United Nations’ announcement was made against a backdrop of heightened political and social tension across Europe, where migration has helped strengthen local economies by increasing consumer demand while also creating additional pressure on state institutions and stoking the popularity of far-right parties and causes. Far-right politicians now share power in Italy and Austria, and have polled unusually well in Sweden and Germany, where thousands of far-right sympathizers rallied in the eastern town of Chemnitz this week in protest over immigration.