LONDON (AP) — Senior Conservative Party members in Britain have intensified their attacks on Prime Minister David Cameron with an open letter accusing him of misleading the public over immigration.

Former London Mayor Boris Johnson and Justice Secretary Michael Gove used a Sunday Times letter to say Cameron's goal of lowering immigration cannot be achieved while Britain remains in the European Union.

The attack on Cameron intensifies the party's internal conflict ahead of a June 23 referendum on whether Britain should remain in the 28-nation bloc.

Johnson and Gove said Cameron had broken promises to lower immigration, corroding "public trust in politics." They are leading the "Leave" campaign in opposition to party leader Cameron.

Cameron's office called the letter "a transparent attempt" to distract voters from the economic costs of leaving the EU.