European Commission officials are to visit the border between Spain and Gibraltar to assess the legality of checks on traffic that sparked a diplomatic row with Britain.
The searches, imposed by Madrid during a dispute over an artificial underwater reef, have led to long border queues, protests and even allegations of violence by commuters against Spanish frontier officials.
The commission said it would carry out a "technical fact-finding mission" in Gibraltar and La Linea in Andalusia after complaints were raised by ordinary citizens, MEPs and the Gibraltar government over the checks on traffic in and out of the British overseas territory. It will also examine claims of smuggling across the border made by Spain.
The Gibraltar government has accused Madrid of a "crude" attempt to "disguise and conceal" the way it conducts the searches ahead of the officials' visit. Last week it said Spanish customs staff reorganised the traffic flow on its side of the frontier to allow a smooth movement of vehicles.
"This crude attempt at the last minute by the Spanish authorities to disguise and conceal the way in which they normally conduct such searches at the border, in a manner that is likely to mislead the European Commission, will be brought to the immediate attention of the European Commission," a Gibraltar government spokesman said.
"Clearly, it would seem that the announcement of the European Commission's visit is already producing positive effects, even before the commission inspectors arrive. The searches conducted ... are in stark contrast to what the Spanish authorities have always done, which is to search cars on either the red or green channels and holding back the rest of the traffic whilst the search is being conducted."
Spain imposed strict checks on traffic at the border with Gibraltar in protest at the creation of an artificial reef. It said the 74 concrete blocks on the seabed disrupt an area used by its fishing boats, while the Gibraltarians said it was a necessary environmental measure.
There have been reports of increasing violence at the border since the end of August as angry commuters face long queues to get to homes in Andalusia. As well as reports of missiles being thrown at Guardia Civil officers, two unions organised a protest after reports that an officer was injured by a Gibraltar-registered car that failed to stop.
The protest prompted calls for renewed efforts, involving the European Union, to solve the diplomatic dispute which has seen Madrid introduce additional checks at the border in protest at what it sees an encroachment on its fishing rights, leaving workers and tourists facing hours in queues to get through.
The border visit will be made by officials from the commission's departments for home affairs, customs union and justice, plus the European Anti-Fraud office (OLAF).