Standard & Poors' threat to cut credit ratings across the euro zone has rung alarm bells and may help French President Nicolas Sarkozy and German Chancellor Angela Merkel force changes to the European Union treaty at a summit this week.
Here are details of what some EU nations would have to do to enable changes to the Lisbon Treaty:
* AUSTRIA: - Any EU treaty changes that require amendments to Austria's constitution would have to be adopted with a two-thirds majority in parliament. Officials have said any move toward creating a fiscal union in the EU would trigger a referendum.
- Parliament ratified the Lisbon Treaty in April 2007.
* BELGIUM: - A treaty change of the magnitude of the Lisbon Treaty needs to be approved by 9 parliamentary bodies.
- A spokesman for the Belgian federal parliament said that in the past this complexity had not been an issue as both the Lisbon treaty and the European constitutional treaty, which was later blocked by referendums in France and the Netherlands, were ratified in time. A referendum is not possible in Belgium.
* BULGARIA/ROMANIA: - In both Romania and Bulgaria only a simple parliamentary majority is needed to back any EU treaty change. Both countries passed the Lisbon Treaty quickly and with little debate.
* CYPRUS: - A parliamentary vote is required and is normally a formality. The Lisbon Treaty was swiftly approved.
* CZECH REPUBLIC: - The Czech Republic would need parliamentary approval for a treaty change. It would also have to be signed off by eurosceptic President Vaclav Klaus. Klaus legally cannot refuse to sign a treaty approved by parliament, but the constitution does not prescribe any deadlines, so he could stretch out the process.
- Klaus had refused to sign the Lisbon Treaty and was the last EU leader to sign in November 2009. Klaus's second and final term runs out in February 2012.
* DENMARK: - Under Denmark's constitution, international treaties can be entered into only with the consent of parliament. Denmark ratified the Lisbon Treaty by passing a regular bill in parliament in April 2008, without a referendum.
- If there were to be a transfer of authority to EU federal institutions, then a referendum would be needed in Denmark after the legislation is adopted in parliament.
* ESTONIA: - Estonia needs a parliamentary vote for amendments to the treaty. The Lisbon treaty took four months to be passed by Estonia's parliament.
* FINLAND: - The Finnish parliament plays a strong role in decision-making on EU affairs compared with many other member states, and a treaty change would require approval by a majority. However, parliament's position on EU issues are generally decided by the Grand Committee. Finland's parliament ratified the Lisbon Treaty in June 2008, with 150 of 200 parliamentary votes in favour.
* FRANCE: - In France, the president can decide whether to submit a vote on changes to the Lisbon Treaty to parliament - known as a "congress" - or to a referendum. After the French rejected the EU constitution in 2005, the president moved the vote to the parliament, which approved it.
- After coming to power in 2007, President Nicolas Sarkozy decided to opt for a parliamentary ratification to avoid the risk of a second rejected referendum.
* GERMANY: - To ratify any treaty changes, both Germany's lower and upper houses of parliament must give their approval and the president must then rubber stamp the legislation.
- Germany was among the last four EU states to ratify the Lisbon Treaty in September 2009, when legislation was delayed for more than a year following an appeal to the country's Constitutional Court by a group of lawmakers.
* GREECE: - Greece ratifies international treaties, including ones on EU matters, in parliament. Theoretically, they can also be ratified by referendum but this has never happened.
- Greece ratified the Lisbon Treaty in June 2008.
* HUNGARY: - To approve any possible EU treaty change, the government submits a bill to parliament, which will have to be endorsed by two-thirds of all lawmakers to become law. Hungary became the first of the 27 EU countries to ratify the treaty in a parliamentary vote on Dec. 17, 2007.
* IRELAND: - The Irish are entitled to vote on fundamental changes to EU treaties and the government has admitted it would struggle to get new reforms past an electorate still stung by Europe's hardline stance on the cost and make-up of Ireland's EU-IMF bailout. The government has said it will seek legal advice about whether a referendum will be needed when it has more details of the proposed changes.
- Irish voters held up the implementation of the Lisbon Treaty when they rejected it in 2009. It was passed in 2010.
* ITALY: - Approval of any change to a treaty would be through a vote in both houses of parliament. Italy approved the Lisbon Treaty in July 2008 through a unanimous vote in the lower house of parliament. The Senate had approved the treaty a week earlier. Formal ratification occurred on Aug. 8. It was the first time that a European treaty had been approved unanimously in both houses of parliament.
* LATVIA: - Changes to treaties require a parliamentary approval. However, the constitution says that more than half of the 100-member parliament can demand a referendum on any "significant" changes to treaties that guide Latvia's participation in the European Union.
* LITHUANIA: - Treaty changes are approved by parliament. Parliament ratified the Lisbon Treaty in May 2008 after just two months. However an official said that depending on what is proposed, Lithuania may need to alter the country's constitutional law which needs a bigger majority.
* NETHERLANDS: - A treaty change will have to be put to a parliamentary vote, but would not require a referendum. The Lisbon Treaty was put to a vote in parliament in 2007.
* POLAND: - Poland does not require a referendum to change the EU Treaty as long as the change does not force an amendment to the constitution.
- The change is very likely to be endorsed by Prime Minister Donald Tusk's centrist, pro-European cabinet. It then requires the parliament's approval before President Bronislaw Komorowski - a close Tusk ally - signs it into law.
- The same procedure took place when the Lisbon Treaty was approved in Poland.
* PORTUGAL: - There is no formal requirement in Portugal for a treaty change to be put through a referendum. However the government can decide at any point to hold a referendum, seeking greater legitimacy for controversial or sensitive measures.
- The Lisbon Treaty only required parliamentary approval and passed easily in April 2008.
* SLOVAKIA: - Parliament needs to approve any changes in international treaties with a constitutional majority, which means at least 90 votes out of total 150.
- Slovakia approved the Lisbon Treaty in April 2008 after weeks of political bickering among opposition and coalition parties over domestic non-related issues.
* SPAIN: - The Lisbon treaty was ratified by Spain's lower house of parliament in June 2008 and then by the uppper house in an extraordinary session in July 2008. Royal approval was given days later on July 30, 2008. A parliamentary vote would be needed for any changes as a result of the Dec. 9 summit.
* SWEDEN: - Parliament would need to approve any changes to the EU treaty. The sitting coalition government does not have a majority and would need opposition support to pass such a change. Sweden approved the Lisbon Treaty in a parliamentary vote in November 2008.
* UNITED KINGDOM: - The European Union Act, which came into force in July 2011, states that any proposed future EU treaty or amendment to existing treaties and ratchet clauses that transfer competences or power from the UK to the EU would require a referendum, although the government has said proposals currently under discussion in Brussels do not fall into that bracket. An act of UK parliament is required before any EU law can take effect in Britain.
- Limited treaty changes which do not transfer powers from London to Brussels must still be approved by an act of parliament before the UK can agree to them - this used to be just a vote in both houses of parliament.
- Britain says a treaty just between 17 countries would not need to be considered by parliament, but one between all 27 in the bloc would. The UK officially ratified the Lisbon Treaty in July 2008 after parliament voted it into law the month before.