How Conservatives and Ukip present their immigration policies will have a strong bearing on the Rochester by-election next month

David Cameron’s plan to curb migration from Europe suffered a major setback today when the outgoing president of the European Commission warned that it would breach EU law.

José Manuel Barroso said that the EU’s free movement rule was “a fundamental principle” that could not be challenged.

His warning comes in advance of a speech by the prime minister in which he is due to set out his proposals for curbing EU migration, which Britain currently has no power to reduce.

Several plans are being considered, including a limit on the number of national insurance cards that can be issued to EU nationals and an emergency brake on numbers.

Mr Barosso, who steps down as the commission’s president at the end of the month, made