A leaked internal document written by Government officials has warned that it is “unclear” whether policies designed to restrict immigration will be effective.

The memo by civil servants at the Department for Education says Government data on immigration remains “poor”.

The quotes are included in a report about a crisis in school places that has now expanded beyond the primary sector. 

The report finds that 35,000 extra places will be needed in secondary schools by 2015.

The report, which was leaked to the Daily Mail and has been obtained by Sky News, is marked “sensitive” with a warning that it “should not be forwarded”.

It says that a baby boom, alongside a surge in immigration since the mid-1990s, has piled the pressure on schools.

Conservative sources blame the previous Labour Government for a lax immigration policy.

But the civil servants go further, raising questions about current policy.

“It is important to note that government data on immigration remains poor and it remains unclear the extent to which current policies will deal effectively with immigration,” the report says.

It adds that EU membership limits what can be done.

David Cameron has promised to reduce net immigration to the “tens of thousands” by placing a limit on the number of visas issued to workers coming from non-EU countries.

A Home Office spokesperson said: “Our reforms are working. We have overhauled the immigration rules, tightening the routes where abuse was rife, and net migration is down by a third since its peak in 2010.

“One of the reasons we need to control immigration is because of its impact on infrastructure and public services.

“We are building an immigration system that works in the national interest. The Immigration Bill, which we will introduce this autumn, is the next step and we will continue to work hard to bring net migration down from the hundreds of thousands to the tens of thousands.”