Prime Minister David Cameron unveils Britain’s EU reform demands next week, firing the starting gun for tough negotiations that London insists will mean changes to the bloc’s treaties.

But can Cameron really expect to rewrite the rulebook ahead of a referendum on Britain’s European Union membership to be held by the end of 2017?

– HOW CAN THE TREATIES BE CHANGED?

The EU is founded on a number of treaties that govern the way the bloc runs, including the original 1957 Treaty of Rome, the 1992 Maastricht Treaty and the 2007 Lisbon Treaty.

They can be changed in four ways: full treaty change, simplified treaty change, decisions of the European Council (which groups EU leaders), and political agreements.

“For treaty change, the British have lowered their ambitions a bit,” Vivien Pertusot of the French Institute of International Relations told AFP.

– FULL CHANGE –

Full treaty change — known as ordinary revision procedure — involves the European Council calling an “intergovernmental conference” of member states, the European Commission (the EU’s executive) and the European Parliament. National parliaments then ratify the changes.

It is very long and complex, and can spark referenda that then shoot down the treaty changes, as happened with the French and Dutch with the European Constitution in 2005.

“Everyone’s now acknowledged that there will not be full-on treaty change,” Pawel Swidlicki, a policy analyst at the Open Europe think-tank, told AFP.

– SIMPLIFIED CHANGE –

Simplified revision — a procedure brought in by the Lisbon Treaty — involves decisions by the European Council that are then ratified by member states, with no intergovernmental conference.

But this can only be used for policies which do not extend the EU’s powers.

It was used in 2011 for the creation of the European Stability Mechanism, the EU’s bailout system.

British Chancellor George Osborne hinted at this in Berlin last week when he called for a “simple mechanism”.

– EU COUNCIL DECISION –

European Council decisions themselves are legally binding, and a decision clarifying provisions in the Treaties could give Britain what it needs in some areas.

The EU used this method to give Ireland guarantees to get it to pass the Lisbon treaty.

– POLITICAL AGREEMENT –

When all else fails, the EU conjures up political agreements that say binding changes will be included in the Treaties at the next opportunity (for example when Treaties are reopened for a new member).

This solution was first used after Denmark voted against Maastricht in 1992.

British officials said in July that because of the tight timescale, Cameron could accept such pledges for amendments after the referendum, instead of immediate treaty change.

– HOW CAN CAMERON GET HIS WISHES? –

Cameron’s demands are in four key areas.

An opt-out from the bloc’s drive for “ever-closer union” could be done through a council decision clarifying that it is not something that applies to all member states, said Swidlicki.

Safeguards for countries that are not in the euro could be part of a broader package of eurozone integration, he said

A “red card” for national parliaments to overrule decisions by Brussels could be done through political agreement, Commission Vice President Frans Timmermans has hinted. The EU already has a yellow card system.

Curbing welfare for EU migrants is the toughest element as it could clash with the EU’s fundamental principle of freedom of movement to work. But a series of recent European Court rulings have appeared to back Britain’s stance on limiting benefits.

In any case, the British government is likely to “put a lot of spin” on whatever concessions it does make so it can sell them to voters, said Pertusot.

– WHAT DOES EUROPE THINK? –

Cameron’s negotiating mandate in the face of sceptical EU partners opposed to “a la carte” EU membership has been strengthened by winning a majority in elections in May.

European Commission President Jean-Claude Juncker has promised to work for a “fair deal” with London, stressing that the EU needs Britain to stay in.