Clegg: Influx of migrants is affecting our schools

He argued many schools were not able to deal with an influx of children who do not speak English or have a poor grasp of the language. Mr Clegg's decision to use immigration as an issue days before the local elections and the London mayoral poll will raise eyebrows at Westminster.

It may also be seen as a sign that he is concerned the Lib-Dems may fare badly in the 1 May polls. Some activists may be alarmed Mr Clegg is shifting to the Right to counter Tory leader David Cameron's appeal to many liberal voters.

Mr Clegg told the 4Children conference in London today that almost 800,000 pupils do not have English as their first language. "We must acknowledge that rising migration is putting pressure on schools at all levels," Mr Clegg said.

"Even a few children in a class can be a real challenge for a teacher used to strong English language skills, especially if children are arriving in the middle of a school year. It's a challenge for native English speakers, as well, because their learning suffers too when a class can't move forward together, learn together, share experiences fully."

He insisted the problem stems from a failure to plan for population changes and not migration itself.