According to those accounts, Borrell told his audience that Trump had compared the situation in northern Africa and the Mediterranean to that along the US border with Mexico. During his campaign, Trump had promised to build a wall along the southern border with Mexico and force the Mexican government to pay for it.

Borrell said that he did not support a wall along the border between the United States and Mexico and that Spanish diplomats had told Trump that the situation was considerably different in scale anyway. Trump disagreed, according to Borrell’s recollection, adding that “the Sahara border can’t be bigger than our border with Mexico”.

The White House did not immediately respond to a request for comment.

Though estimates vary, US Customs and Border Protection says that the border between the United States and Mexico is roughly 3,000km long. Most estimates for the Sahara put it at about 4,800km from east to west.

A wall across the Sahara would probably also involve building through several sovereign nations. The desert spreads over parts of 11 countries: Algeria, Chad, Egypt, Eritrea, Libya, Mali, Mauritania, Morocco, Niger, Sudan and Tunisia.

It was unclear from the Spanish media whether Trump was speaking in jest when he made the purported comments. Borrell had accompanied Spain’s King Felipe VI and Queen Letizia to the White House in June.

Spain has seen rising numbers of migrants and refugees arrive on its shores in recent months, the result of a crackdown on a central Mediterranean route between Libya and Italy. According to the International Organisation for Migration, more that 33,000 people have arrived in Spain via the western Mediterranean in 2018 - three times as many as in the same period last year.

The left-wing government of Prime Minister Pedro Sánchez, in power since June, has attempted to welcome the new arrivals - accepting migrant rescue boats such as the MV Aquarius after Italy rejected them.

Borrell, who was president of the European Parliament between 2004 and 2007, has been critical of anti-immigration sentiment in Europe. “Everybody in Europe is being affected by this virus, fears of immigration,” he said during the June visit to the United States. “This is not the case of Spain.”

His comments at the luncheon this week appear to affirm, however, that migration and identity are the most important issues facing the continent.

“European societies are not structured to absorb more than a certain percentage of migrants, especially if they are Muslims,” he said, according to El Pais.