New negotiation of a previous agreement.

The leader of the British Conservative party has promised his party a re-negotiation of some parts of the Lisbon Treaty. David Cameron wants different opt-outs and will have a referendum on continued EU membership in 2017 - if he is re-elected.

Originally, he wanted a referendum on the Lisbon Treaty as promised by Tony Blair and Gordon Brown. But it was decided before he won the elections in the UK.

When the last signature was delivered 3 November 2009 and the Lisbon Treaty entered into force 1 December 2009 this possibility was lost. A new treaty requires unanimity between all member states.

Opt-outs also requires unanimity but is easier to negotiate. They can be linked to any treaty amendment, also those on accession of new member states. Ireland and the Czech Republic was offered new Protocols to be inserted in the first coming treaty amendment.

In 1974, the British Labour Government insisted on re-negotiating Britain's membership conditions. However, this re-negotiation did not change the Accession Treaty governing Britain’s membership of the EEC. Afterwards, in 1975, Britain held a referendum on continued membership of the EEC. A majority of voters approved. 

Later, in 1984, the British Prime Minister 
Margaret Thatcher negotiated a special rebate on Britain’s financial contributions to the EC to take account of what she considered her country’s excessive payments. 

Spain also renegotiated certain provisions after membership, although any change in the actual wording of a country’s Accession Treaty is legally impossible once it has been ratified and come into force. 

Applicant countries may seek to make changes in the interpretation of their Accession Treaties after they have joined the EU, but changes in the stated provisions and wording of the treaties are only possible if adopted by unanimity.

Before joining, the EU applicant countries have to negotiate every concession regarding the application of the Acquis Commaunautaire with the EU Commission. Concessions must also be agreed with every EU state, all of whom must ratify the Accession Treaties. This makes it very difficult to negotiate flexible conditions, 
derogations, or opt-outs from the 100.000 page EU-law.

After becoming members, it is much easier for member states to negotiate. Then they have the right to vote and can threaten to block laws requiring unanimity - powers they did not have when negotiating accession. It is therefore expected that problems not solved during the accession negotiations will reappear.