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Saying no to European legislation is not a simple process; now David Cameron wants to make it a whole lot easier.

National parliaments can currently voice opposition to new EU rules, but lack the power to strike them down on their own.

But the prime minister has said a key part of his EU renegotiation strategy will be reforms to give parliaments powers to actually block unwanted EU laws.

Image copyright Camilla Svensk / Rikstag Image caption The Swedish Rikstag has objected to more EU laws so far than any other national parliament

The UK is not the only country where there have been calls for increased blocking rights, with other parliaments also demanding a greater say over EU legislation.

So how does it currently work?

Currently, the so-called "subsidiarity control mechanism", introduced in 2009, allows parliaments to formally accuse the Commission of overstepping its remit.

What is subsidiarity?

Subsidiarity is the principle that legislation should not be created at an EU-wide level if the issue can be addressed more effectively by national laws

The principle only works in areas where the EU and member states share responsibility

However, it has been little used so far, with only a small number of EU laws attracting attention from a substantial number of parliaments.

In fact, of the 125 laws that have been questioned using the procedure, only 19 have attracted complaints from more than three national legislatures.

Latest figures in the Commission's annual reports also reveal wide discrepancies in the extent to which the current scheme has been used in different countries.

Westminster lawmakers have objected to 22 different EU laws in total, the same number as their counterparts in the Netherlands.

How subsidiarity works

National lawmakers have eight weeks to issue a "reasoned opinion" objecting to a new EU law

Each parliamentary chamber gets a "vote", with single-chamber parliaments allowed two votes

A third of the votes (or a quarter for home affairs measures) triggers a "yellow card" against a law

In this case, the Commission must review the law but is under no obligation to withdraw it

Half of the votes among the parliaments are enough to issue an "orange card"

In this case, the law can be blocked - but only if either half of MEPs in the European Parliament or 55% of national governments agree.

But both sets of parliamentarians have been eclipsed by Swedish legislators, who have accused the Commission of overstepping its powers in 51 pieces of legislation.

In contrast, the procedure has hardly been used at all by parliaments in Slovenia, Latvia, Estonia or Greece.

Criticism of system

The "yellow card" has only been raised twice, and no EU law has ever attracted enough objections to receive an "orange card".

Despite this, critics of the system have argued that use of the system so far does not reflect the number of concerns about the scope of EU legislation.

The treaty rules only oblige the Commission to provide a written response to complaints, justifying why a set of proposals meet the bloc's rules on "subsidiarity".

This principle states that the Commission should not legislate at an EU-wide level if the issue can be addressed more effectively by national laws, in those areas where the EU and member states share responsibility.

However, it does not apply to areas where the EU has exclusive rights to legislate, including rules relating to customs union and the internal market.

Additionally, the concept - originally drawn from an organising principle of the Catholic church - has proved hard to define in practice.

The UK government has also argued that the current mechanism is too difficult to satisfy, with the threshold for triggering a yellow card too high and parliaments not given long enough to respond to new EU proposals.

Last year, it endorsed an extension to the eight-week deadline suggested by the EU committee of the House of Lords.

'Not achievable'

Ahead of the upcoming referendum, David Cameron has said he wants national parliaments to go further, and be able not just to query, but to block, unwanted EU laws.

At the moment, even an "orange card" cannot take a law off the table without the additional approval of half of MEPs or national governments.

Former foreign secretary William Hague had already outlined plans for a "red card" blocking system in 2013, telling the Königswinter Conference that national legislatures, not the European Parliament, remained the "democratic levers voters know how to pull".

Some Eurosceptic Conservatives have even suggested Westminster should have further powers, allowing it to veto EU laws unilaterally.

That demand will not be on the table during EU renegotiations, however, having been dismissed as "not achievable" by Foreign Secretary Philip Hammond.

Image caption Philip Hammond told the BBC in June that a UK veto of EU laws would not be possible

Speaking in June, Mr Hammond said such a power would "effectively be the end of the European Union".

Supporters of reform say Commission reaction to criticism of 2013 plans to set up an EU-wide body to investigate fraud of the EU budget shows the need for a stronger system.

A total of 11 parliaments - including Westminster - voiced concerns about the measure, but the plans remain on the table, with the Commission maintaining that it does not breach "subsidiarity" rules.

When have 'yellow cards' been issued by parliaments?

The first proposal to be shown the card was in 2012, when parliaments clubbed together to oppose common EU rules on the right to strike.

In this case, the Commission did eventually decide to withdraw the proposals voluntarily - although it denied that it had acted beyond its remit.

The second card came in 2013, when legislatures objected to plans to create a European Public Prosecutor's Office (EPPO) for prosecuting cases of fraud against the bloc's budget.

A total of 18 votes were issued by parliaments, but the Commission maintains that the plan is justified.

However, the figures show that that comparatively few laws have attracted the same level of formal objection.

In the same year as the yellow card against the EPPO proposals, the next most objected-to law was a planned EU directive on common coastal management - which received only nine votes from national chambers.

'Quality' of legislation

EU legislation on cigarette packaging, forcing larger picture warnings on the front and back of packets, received seven objections - not enough to stop the law, which was adopted in April last year and will have to be translated into national laws by next May.

In the whole of last year, even the most controversial proposals only received formal challenges from three parliaments.

The UK government has not yet said at what level it thinks the threshold to block a law should be set.

However, a report published under the previous coalition predicted that such a tool would likely only be used "very rarely", suggesting that a red card would would be used mainly to improve the "quality" of EU legislation.

Image copyright Getty Images Image caption EU laws on cigarette packaging will come in to force next year despite initial objections from some parliaments when the plans were announced

The UK's ideas chime with proposals that have also been suggested by some European parliaments, although it remains to be seen how many would support plans for an EU "red card".

The European Commission has said it sees the need to give parliaments a greater role, although proposals to change the bloc's treaties have so far not been proposed.

Whatever is eventually agreed, what remains unclear is how far reforms would need to go to achieve meaningful change, or how parliaments and governments would co-operate in sharing the power to block new laws.

Or indeed, whether a renegotiation of parliaments' powers will change the public's opinion of EU legislation in the UK.