Europe Minister David Lidington called for "real action" from Spain over delays at its border with Gibraltar as the European Commission sent inspectors to assess progress in easing queues which the UK blames on stricter controls imposed as part of a diplomatic spat.
Brussels set a six-month deadline in December for both sides to tackle huge queues caused by the changes, which were put in force in apparent retaliation for the building of a concrete reef by Gibraltar that Madrid says was a deliberate threat to its fishing industry.
Officials have now returned to The Rock to examine what work has been done to speed up border crossings and deal with Spanish concerns over tobacco smuggling, which it says are the reason for the crackdown.
The fresh inspection was one of the demands made by MPs in a report yesterday which criticised the UK Government for sitting "on the fence" in the latest bout of tensions over Spanish claims to sovereignty over the territory, which Britain has held since 1713.
The Commons foreign affairs committee called for a more "robust" response - including the open threat of legal action against the Spanish government and police and military options for countering Spanish incursions into British territorial waters.
Mr Lidington, who has just returned from a visit to Gibraltar, said: " Delays at the border in recent weeks experienced by the people of Gibraltar and the Campo area were at totally unacceptable levels.
"I am pleased that the Commission responded to our lobbying for a return by the border monitoring mission. The UK Government has regularly raised border delays with the Commission since the beginning of disruption last summer, and we will continue to do so.
"I visited Gibraltar this week and saw at first hand the positive and decisive steps which the government of Gibraltar has taken to address the recommendations the Commission made to them seven months ago.
"It is now time to see real action from the Spanish authorities to implement the recommendations made to them, and for them to end the current regime of disproportionate checks.
"The UK Government expects the Commission to ensure that the Spanish authorities act on the Commission's recommendations and return border checks to reasonable and proportionate levels. Gibraltarians are citizens of an EU member state and their European rights must be fully respected."