Lord David Cameron has warned that "every inch" of Gibraltar remains part of Britain amid new EU border rules.
The Foreign Secretary said the UK’s sovereignty over Gibraltar "won’t change."
It comes as talks about the territory’s relationship with Brussels are ongoing between the EU, Britain and Spain.
A final agreement is expected to be announced within the coming weeks.
Speaking to the European Scrutiny Committee, Lord Cameron said: "I think I understand the meaning of sovereignty which is Gibraltar is a sovereign UK territory and that shouldn’t and won’t change and in our negotiations there are three things that are absolutely paramount.
"One is the sovereignty of Gibraltar, the second is the operation of the defence facility and the third is trying to achieve a deal which is good for the people of Gibraltar and crucially backed by the government of Gibraltar.
"So those are our aims but we don’t want to see anything that diminishes in any way UK sovereignty over Gibraltar.
He added that "every inch of the Rock remains British."
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Gibraltar has been a British territory since it was handed to the UK as part of the Treaty of Utrecht in 1713.
There have been concerns that under a treaty, EU Frontex border guards decide who can enter the British overseas territory – and will give them the power to turn away British citizens.
Any potential treaty with the EU could cause further concerns because of implications of a dilution of British sovereignty in areas such as Northern Ireland.
Pro-Brexit DUP committee member Sammy Wilson told The Independent: "Brexit was supposed to be about taking back control. It is proving to be the opposite."
Lord Cameron confused some MPs by saying he supports the Gibraltar government’s push for a "fluid border" which would have it operate as an adjunct to the Schengen area under free movement rules.
He insisted that this meant EU checks "will need to take place" in Gibraltar but likened it to Eurostar border checks by the French at St Pancras station in London.
Conservative MP for Peterborough Paul Bristow said: "You would accept that a crucial difference is that while the Spanish claim sovereignty over Gibraltar the French do not claim sovereignty over St Pancras yet."
The foreign secretary was also unable to rule out access to Gibraltar by Spanish police "in hot pursuit" or for surveillance purposes which are part of the Schengen accords. He added: "These are all part of the discussions."
