The Spanish ships which entered British waters off Gibraltar acted in an "unlawful" and "dangerous" way, the Foreign Office has said.
"Not only were the actions of the survey vessel unlawful, but it was accompanied by a Spanish Guardia Civil vessel whose dangerous manoeuvring presented a significant safety concern on the waters," Europe minister David Lidington said.
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The vessel was spotted off the Kent coast at around 5.40am on Thursday.