A diplomatic row over Defence Secretary Geoff Hoon's visit to Gibraltar has escalated, according to reports.
Gibraltar is celebrating its 300th anniversary
The Spanish media says Foreign Secretary Miguel Angel Moratinos is disappointed Mr Hoon will visit for the territory's 300th anniversary party.
UK military personnel will take part in the ceremonies on Wednesday.
The Ministry of Defence is refusing "for security reasons" to confirm
whether or not Mr Hoon will travel to the Mediterranean outpost.
Gibraltar has been a source of dispute between London and Madrid since it was seized by Britain in 1704.
Relations 'not easy'
On Friday Mr Moratinos told a news conference it was "not easy to maintain normal relations with Great Britain" because of the Gibraltar issue.
Deputy Prime Minister Maria Teresa Fernandez de la Vega said a visit by Mr Hoon would be "inopportune".
The row is the third time in three months Anglo-Spanish relations have been strained over Gibraltar.
In June there were protests over a visit by Princess Anne
and in July a port call by the British submarine HMS Tireless drew criticism.
The Foreign Office has confirmed British Ambassador Stephen Wright had met Mr Moratinos, but refused to discuss the content of their conversation or reveal whether the meeting had been at Spain's request.
Sir Menzies Campbell, foreign affairs spokesman for the Liberal Democrats said: "As fellow members of NATO and the EU, Spain has no need to react in this way.
"By doing so the Spanish Government simply confirms the worst fears and prejudices of those in Gibraltar who are determined that there should be no accommodation with Spain."
The tercentenary celebrations began on Saturday with the arrival of HMS Grafton in Gibraltar following a tour of duty in the Persian Gulf.
On Wednesday a military parade will be held and the Royal Navy granted freedom of Gibraltar.