Estimates of the largest Muslim populations in Western Europe as a percentage of total country population:

7.0% France
3.9% Sweden
3.4% Germany
3.4% Belgium
2.7% Britain see Muslims in Britain
2.0% Netherlands
2.0% Denmark
1.6% Norway
1.4% Italy
1.1% Spain

(Source: The Economist, April 3, 2003)
The figures are minimum estimates, and not necessarily exactly comparable, due to differences in method of data collection and data presentation in different countries.