GENEVA (AFP) - First and second generation immigrants now make up more than a third of Switzerland's inhabitants over the age of 15, a significant increase in the past decade, statistics showed Thursday.

Most of the newcomers are European, with Italians and Germans making up the largest percentage at 11 percent each, followed by people of Portuguese and French origin.

In 2013, 2.4 million of the 6.8 million people above the age of 15 living in Switzerland had a migrant background, up from 1.7 million in 2003, according to data published by the Federal Statistical Office (FSO).

A full 80 percent were born abroad, while the remaining 20 per cent were born in Switzerland to immigrant parents.

More than one third hold a Swiss passport, FSO said.

Getting nationality can be a lengthy process in Switzerland, and is not guaranteed to second or even third-generation immigrants, although discussions are underway to make it easier for grandchildren of migrants to receive the coveted Swiss passport.

Immigrants with no blood-ties to Swiss nationals through birth or marriage are meanwhile required to live in the country for at least a decade before applying, and even then must prove they are well-integrated and well-versed in Swiss languages, laws and customs.

The influx is having a clear impact on demographics in the wealthy Alpine nation, since the migrant population is significantly younger than their native counterparts, FSO said.

"This younger population is slowing down the ageing of the resident population in Switzerland," it said.

The statistics did not provide the same break-down for Switzerland's 1.2 million children under the age of 15, due to incomplete data.

FSO said around 90 per cent of these children had been born in Switzerland, but it remained unclear how many of them were second generation immigrants.