The European Union has received some 7,000 complaints about an increase in controls at La Linea on the Spanish side of the border since February.

People have had to wait between two and four hours to cross from Gibraltar into Spain, despite the fact that it is an EU internal border.

EU Internal Market Commissioner Mario Monti believes that the Spanish measures are "disproportionate" and hinder the free movement of people and goods guaranteed under EU internal market rules.

However, Spanish EU commissioner Marcelino Oreja argues that because Gibraltar is not part of the Customs Union or the Schengen visa convention, Spain must "systematically check" people and goods crossing the border.

History of tension

Any ruling will only increase the tension between the UK and Spain over the status of Gibraltar.

The tiny territory was ceded to Britain under the Treaty of Utrecht in 1713, and Gibraltarians are adamant that they want to maintain their status as a UK Overseas Territory.

In February, Gibraltar seized a Spanish fishing boat apparently inside its territorial waters, and Spain retaliated with tighter frontier controls.

When UK Prime Minister Tony Blair met his Spanish counterpart Jose Maria Aznar in April, they side-stepped the issue.

Meanwhile, the British Government has backed Gibraltar's complaints over the border go-slow, while Spain is currently blocking at least five pieces of EU legislation because of the row over Gibraltar's status.

As well as considering complaints about border controls, the European Commission is also likely to take action against the UK for failure by Gibraltar to adopt four pieces of EU company legislation.

An official insisted the fact the decisions against Britain and Spain were likely to be taken at the same time was "a coincidence".