David Cameron has spelled out to ministers that they will not be able to campaign for a British exit from the European Union until after the negotiation process is complete.
The Prime Minister called for ministers on both sides of the argument to "treat each other with appropriate respect and courtesy" in a personal minute setting out the "wholly exceptional" rules which will allow members of the Government to support Brexit once the talks have finished.
The "special arrangement" will allow individual ministers to take a different position from the official Government line after a Cabinet discussion, which will follow the conclusion of talks between Mr Cameron and his fellow EU leaders.
The Prime Minister hopes to strike a deal on his demands at a crunch summit in Brussels next month which will then allow him to recommend that the UK remains within a reformed EU, but until the talks are concluded he stressed that all his ministers should follow the Government line.
In the minute sent to ministers, the Prime Minister said: "Until that point - when it will become clear whether a deal can be negotiated that delivers the objectives I have set out - 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."
Officials would be expected to support the Government's stance and "it will not be appropriate or permissible for the civil service or individual civil servants to support ministers who opposed the Government's official position".
But ministers who oppose the official position would be able to draw on personal help and advice from their special advisers, known as spads, as long as it is in line with their wishes and "in their own time".