In his letter, Mr Cameron said: "All ministers should continue to support the position set out in our manifesto and say or do nothing that will undermine the Government's negotiating position."
He adds: "It will be very important during this period [the final 28 days before the referendum] for ministers on both sides of the debate to treat each other with appropriate respect and courtesy.
"We all recognise that the UK's membership of the EU is an issue on which some colleagues have long-standing, deeply-held positions of conviction - hence my decision to set aside, on a highly exceptional basis, the normal rules of collective responsibility. But throughout this period, and in its aftermath, we will continue to have responsibility for governing the country and serving the public who elected us. This can only be done effectively if we remain, despite differences on this one issue, a united, harmonious, mutually respectful team."
David Cameron's full letter to ministers
In the letter, Mr Cameron also suggests that any minister hoping to make any argument in favour of Brexit will have to do so from the backbenches.
Source: http://telegraph.feedsportal.com/c/32726/f/579309/s/4cc94222/sc/13/l/0L0Stelegraph0O0Cnews0Cnewstopics0Ceureferendum0C120A933120CEU0Ereferendum0EDavid0ECameron0Ewarns0Eministers0Enot0Eto0Eundermine0Erenegotiations0Bhtml/story01.htm