Factbox: How EU offer to Britain's Cameron is shaping up

(Reuters) - A draft EU reform package to help keep Britain in the European Union could be circulated on Monday following meetings between Prime Minister David Cameron and top EU officials. The following are key points of what Reuters has been told by sources close to the negotiations could be the proposal European Council President Donald Tusk will send to EU governments after talks over dinner with Cameron in London on Sunday: THE FORM Negotiators will work through the weekend to craft a single document laying out legislative and other measures responding to Cameron's November demands for reforms so he campaigns to keep Britain in the EU in a referendum by the end of next year. Depending on how Friday's talks in Brussels with European Commission President Jean-Claude Juncker have gone, and on the Cameron-Tusk meeting, the document could set out in more or less detail a classic EU negotiating text, including blank spaces and alternative wordings in brackets, to be haggled over up to and during a summit chaired by Tusk in Brussels on Feb. 18-19. Nothing is done until everything is. A deal, needing all 28 national leaders' assent, could pave the way for a referendum as early as June. Impasse would probably mean more talks. The EU sees its proposals as legally watertight, safe from challenges in court and not requiring amendments to treaties now - something hard to pull off quickly across all 28 EU states. On some issues, it will offer binding guarantees that treaties will be amended later if that is required to enact proposals. MIGRATION Cameron wants to discourage other Europeans coming to Britain by excluding them from the tax credits, child allowances and other non-contributory social benefits attached to low paid British jobs for at least four years. Without changing EU treaties guaranteeing free movement of labor and barring national discrimination, EU lawyers propose an "emergency brake", limiting those fundamental rights where vital national interests or economic stability are at risk. Legislation would give any state to curb in-work benefits for up to four years - if agreed to the European Council of fellow governments. Normally, Council decisions are by consensus - in effect, unanimity - but easier terms might be negotiated. Allowances for children could also be reduced long term. EU negotiators speak of "indexing" so that workers whose children live in cheaper states than the parent would receive less. EURO ZONE Cameron wants more legal safeguards for Britain's sterling-based economy and big financial industry from a risk of the euro zone countries writing EU rules to suit them. The EU proposes another "emergency brake" where Britain could object in the Council of all EU ministers. How far Britain may block measures and how far it would need allies is unclear yet. London and euro zone leaders all say Britain should not have a blanket veto. NATIONAL SOVEREIGNTY Cameron wants assurances Britain need not hand more power to Brussels and to enhance the say of nations in the EU. The EU will be proposing a "red card", letting national parliaments acting in concert block EU legislation. How few legislatures could obstruct how much is still being negotiated. Britain wants it made clear that an EU treaty phrase calling for "ever closer union" among peoples does not mean more political integration. The EU will offer a binding decision by the European Council, echoing a reassurance it gave in 2014. COMPETITIVENESS The least contentious area of Cameron's four reform "baskets", calling for less red tape and more economic dynamism has broad backing so a set of declarations will echo EU policy, but with elements to show Britain Brussels is listening. (Reporting by Paul Taylor and Alastair Macdonald; Writing by Alastair Macdonald; Editing by Lisa Shumaker)