MEPs will take the next crucial step in the four-year battle over software patents tomorrow with a decisive vote on the offending directive.

Intended to harmonise the patent regime across all 25 EU member states, the directive on computer-implemented inventions (CII), has proved extremely controversial, pitting the Parliament against the Council of Ministers.

MEPs have to reach an agreement with the Council on the text of the legislation before it can become law, and tomorrow, Wednesday6 July, they will consider a series of amendments agreed by the Council in May. However, the Council has previously rejected amendments put forward by the Parliament, opening the way for yet more disagreement.

While all sides agree that software itself should not be patentable as it is covered by copyright law, the fierce debate over the directive has been over the wording of the legislation and, in particular, how terms like invention and innovation are defined.

Large IT companies argue that some of the amendments proposed by MEPs do not offer sufficient patent protection because they are based on out-dated definitions. Critics, mainly from the open source community but also including IBM and Sun, argue that Council amendments would open the door to widespread patenting of software and computer programs and allow large IT firms to dominate the market.

A large number of MEPs want to clarify definitions to ensure that there are no loopholes for patent lawyers to exploit which would allow software patents through the back door.

A group of almost 200 MEPs from across the political spectrum, including representatives of the far left, the Greens, the Socialists, some Liberals and center-right politicians, is pushing a package of 21 amendments which seeks to restrict the scope of the patent regime. To be adopted, amendments need the support of at least 367 of the 732 MEPs.

"If we want to preserve a competitive, innovative and successful European IT sector, it is essential that the Council's common position be amended. We believe the set of 21 compromise amendments being put forward by MEPs from all political groups is what is needed to achieve this goal and avoid the worst case scenario of a US-style software patent system," said Rufus Pollock, director of the Foundation for a Free Information Infrastructure UK.

However, large companies including German software developer SAP AG contest this view. "The open source community thinks patents have got in their way. But SAP only has 24 patents. If it was true [that patents got in the way], how come open source has flourished? No one has been sued. Small and medium-size software companies have more patents than SAP," said Les Hayman, an advisor to SAP's CEO.

It's not clear if a sufficient number of MEPs will back the package of amendments, or whether there will be enough support for the legal affairs committee's position. In this case, the Council common position adopted in May will stand.

The legislative process doesn't end with Parliament's vote. The Council then has three months to consider Parliament's amendments. It can accept them, or choose by unanimous vote to reject them. If it rejects them, Parliament and the Council would enter a special process designed to broker an agreement, called conciliation, and if this failed to produce a deal, the directive would be abandoned.