The Foreign Office also denounced as "extraordinary and unprecedented" a threat by the Spanish Foreign Minister Abel Matutes to ban Gibraltar aircraft from flying over Spain.
Last night at least two Gibraltar drivers were stopped as they tried to cross the border by Spanish guards who said their licences were invalid, the Foreign Office said.
A spokesman also said last night that Robin Cook, the Foreign Secretary, had asked Neil Kinnock, Britain's European Union transport commissioner, to intervene in a row which seriously threatens relation between two EU partners.
The Spanish measures appear to stem from a deal struck a fortnight ago between Gibraltar and some Spanish fisherman after the seizure of a fishing boat in Gibraltar waters - which Spain does not recognise. "We have remonstrated with the Spanish authorities tonight," a Foreign Office source said yesterday. "The Gibraltar driving licence and the EU licence should be mutually recognised by EU member states. "On overflights, that is a matter for the Spanish air traffic control, but it would be extraordinary and unprecedented if an EU member state was to impose an overflights ban on another EU member state."
The threats which sparked the row were made yesterday by Mr Matutes in the Spanish parliament.
The Chief Minister of Gibraltar, Peter Caruana, appealed last night to Britain for help. In a statement he said de facto restrictions had been in place for the last week, causing long tailbacks at the frontier. Spain has long accused Gibraltar of being a hotbed of drug money laundering and illegal trading, and periodically tightens the screw to cut down on the alleged activities.
The Spanish are furious at Mr Caruana who, in an unprecedented move invited the fishermen into Gibraltar's government building and made a deal with them, whereby Gibraltar would turn a blind eye, as long as the trawlers did not get too close. Mr Matutes refuses to talk to Mr Caruana, insisting he will deal only with Britain. And the only thing he wants to talk about is sovereignty.
A Spanish proposal for a long period of joint sovereignty, during which British and Spanish flags would fly over the rock before the colony eventually joined Spain, has languished in Mr Cook's in-tray for more than a year. Some EU colleagues urge him to look favourably on the proposal, but he has promised Mr Caruana not to move without Gibraltar's consent.
The Tory shadow Foreign Secretary, Michael Howard, last night called on Mr Cook to "break his wall of silence over this issue and ensure that this episode is not allowed to escalate further into a full-blown diplomatic crisis. We will be tabling parliamentary questions and calling for a clear plan of action."