Illegal immigrants, now estimated to make up a fifth of the population of Seine-Saint-Denis, north-east of Paris, are severely straining public services and creating social tensions, according to a parliamentary report.

Seine-Saint-Denis has long been the French department with the highest proportion of immigrants, but the report warns that the number of illegal migrants may have risen as high as 400,000.

The report catalogues what the conservative newspaper Le Figaro describes as “the incredible deterioration of social, economic and security conditions” in the area, where 28 per cent of the population lives below the poverty line.

Police, schools, courts and other public services are struggling to cope, while the presence of undocumented foreign nationals is blocking the implementation of effective policies by the authorities, argue the two MPs who wrote the report.

Rodrigue Koukouendo, from President Emmanuel Macron’s centrist party, and François Cornut-Gentille from the centre-Right Republicans, are urging the government to review France’s ban on gathering data on the ethnic makeup of the population so that immigrant numbers can be better monitored and regulated.