EU leaders hope 2009 will be the year that the Lisbon Treaty finally comes into effect, after months of delay.

The treaty is aimed at streamlining EU institutions to make the enlarged bloc of 27 states function better. Opponents say it is part of a federalist EU agenda that threatens national sovereignty.

The treaty was rejected by Irish voters in a referendum on 12 June 2008 and, under EU rules, it cannot enter into force if any of the 27 member states fails to ratify it.

Signed in Lisbon in December 2007, the treaty was drawn up to replace the draft European constitution, which was thrown out by voters in France and the Netherlands in 2005. Opponents, and even some of the constitution's architects, say Lisbon differs little from the constitution.

Can the EU still complete ratification?

Yes, but it is not clear what will happen if Irish voters reject the treaty again in a second referendum this year. That could scupper the treaty, or at least further delay its implementation.

At a December summit EU leaders agreed on a "roadmap" to get round the blockage caused by the Republic of Ireland's No vote.

The Irish government agreed to put the Lisbon Treaty to a second referendum by November 2009, in return for a set of EU "legal guarantees" aimed at addressing various concerns raised by voters. The EU pledges not to impose rules on Ireland concerning taxation, "family" issues - such as abortion, euthanasia and gay marriage - and the traditional Irish state neutrality.

Announcing the new Lisbon deal, French President Nicolas Sarkozy also said that under Lisbon "every member state will have a commissioner" - another concession to Ireland. That promise might prove difficult to reconcile with the original plan under Lisbon to have fewer commissioners than member states, as from 2014.

The Irish "protocol" will be bolted onto Croatia's treaty of accession, which will have to be approved by all 27 member states. Mr Sarkozy said that would happen "in 2010 or 2011".

Mr Sarkozy again ruled out any renegotiation of Lisbon. The only countries that have not yet ratified it are the Czech Republic, Ireland and Poland.

Czech ratification is complicated by the fact that Czech President Vaclav Klaus, a Eurosceptic, does not like the treaty. And the Czech Prime Minister, Mirek Topolanek, says Lisbon ratification depends on whether MPs vote for the government's plan to host a US radar base. But as current holder of the EU presidency the Czech Republic will not want to be embarrassed over Lisbon.

In the UK, the opposition Conservatives want a referendum on Lisbon. If they were to win an early general election they could conceivably delay ratification further.

Hungary was the first of 25 countries to approve the treaty in parliament.

Poland's President Lech Kaczynski has refused to sign the treaty for the time being, calling it "pointless". He says he is waiting for the Irish question to be resolved.

Was Ireland the only country to hold a referendum?

Yes. Most EU leaders argue that Lisbon is an amending treaty which does not transform EU structures to the extent that a referendum is necessary.

That position is rejected by the Irish No camp and the British Conservatives, as well as by many Eurosceptics across the EU.

One of the Irish No camp's leaders, Declan Ganley, says his party Libertas will compete for seats in all EU states in the June European Parliament elections.

According to an Irish Supreme Court ruling in 1987, any major amendment to an EU treaty entails an amendment to the Irish constitution - and that in turn requires a referendum.

How similar is Lisbon to the draft constitution?

It contains many of the changes the constitution attempted to introduce, for example:

• A politician chosen to be president of the European Council for two-and-a-half years, replacing the current system where countries take turns at being president for six months.

• A new post combining the jobs of the existing foreign affairs supremo, Javier Solana, and the external affairs commissioner, Benita Ferrero-Waldner, to give the EU more clout on the world stage.

• A smaller European Commission, with fewer commissioners than there are member states, from 2014.

• A redistribution of voting weights between the member states, phased in between 2014 and 2017 - qualified majority voting based on a "double majority" of 55% of member states, accounting for 65% of the EU's population.

• New powers for the European Commission, European Parliament and European Court of Justice, for example in the field of justice and home affairs.

• Removal of national vetoes in a number of areas.

Most European leaders acknowledge that the main substance of the constitution would be preserved.

If it contains the same substance, why is the Lisbon Treaty not a constitution?

The constitution attempted to replace all earlier EU treaties and start afresh, whereas the new treaty amends the Treaty on the European Union (Maastricht) and the Treaty Establishing the European Community (Rome).

It also drops all reference to the symbols of the EU - the flag, the anthem and the motto - though these will continue to exist.

How long did it take to agree the treaty?

The effort to draft a constitution began in February 2002 and took two-and-a-half years, but that text became obsolete when it was rejected by French and Dutch voters in 2005.

Work began in earnest on a replacement treaty during the German EU presidency, in the first half of 2007, and agreement on the main points of the new treaty was reached at a summit in June that year.

Negotiations continued behind the scenes over the following months before a final draft was agreed by the leaders of the 27 member states in October 2007.

Why was the constitution dropped?

France and the Netherlands said they would be unable to adopt the constitutional treaty without significant changes, following the 2005 referendums.

The UK also pressed hard for a modest "amending treaty", which could be ratified by means of a parliamentary vote, like earlier EU treaties.

Does the Charter of Fundamental Rights feature in the new treaty?

No. There is a reference to it, making it legally binding, but the full text does not appear, even in an annex.

The UK has secured a written guarantee that the charter cannot be used by the European Court to alter British labour law, or other laws that deal with social rights. However, experts are divided on how effective this will be.

Are any countries seeking opt-outs?

Ireland and the UK currently have an opt-out from European policies concerning asylum, visas and immigration. Under the new treaty they will have the right to opt in or out of any policies in the entire field of justice and home affairs.

Poland is also due to sign up to the guarantees on the Charter of Fundamental Rights negotiated by the UK. During the treaty negotiations, Polish leaders voiced concern that the charter could contradict Polish law in moral and family matters.

Denmark will continue with its existing opt-out from justice and home affairs, but will gain the right under the new treaty to opt for the pick-and-choose system.

When will the new treaty kick in?

The pre-referendum plan is in disarray now. Originally, the treaty was supposed to come into force in January 2009.

The schedule - which may well change - currently looks like this:

• The High Representative on foreign affairs will not start work until the treaty has been ratified. The new president of the European Council could also start work at that point.

• The European elections in June 2009 will be held under the existing Nice Treaty. That means there will be 736 seats in the European Parliament - down from the current 785. Under the Lisbon plan, the number will be fixed at 751.

• Although a new European Commission will be chosen in October 2009, its size will not be slimmed down until 2014.

• Some extensions of qualified majority voting in the European Council are already in place, such as the appointment of the new commission president and the High Representative for Common Foreign and Security Policy - but Poland's objections over voting weights mean the redistribution of votes will not come in until after 2014.

It could be at least 10 years before the process is complete.