Argentinian aggression towards the Falkland Islands may be fuelled by a British exit from the European Union, it has been claimed.
Spain would "take advantage" of the UK quitting the bloc to "further undermine, isolate and exclude" Gibraltar, MPs have also been told.
The British Overseas Territories warned the Foreign Affairs committee they feared losing vital protections if voters backed Brexit in the June 23 referendum.
Sukey Cameron, the Falkland Islands government representative in the United Kingdom, said quitting would have "wide-ranging and deep implications".
"Were the UK no longer a member of the EU that support would be much less certain from a large number of those EU member states, and might encourage Argentina to be much more aggressive in its approach," she said.
A statement from the government of Gibraltar said: "In Gibraltar's case, experience has shown that Spain would take advantage of any such renegotiation in order to further undermine, isolate and exclude Gibraltar from the European mainstream."
The warnings were contained in a report by the committee, made up of an equal number of Leave and Remain supporting MPs, on the implications of the referendum.
It "cannot be assumed that the UK would retain full or partial access to the single market" if Britain left or that it would want to "given the restrictions and costs that such an arrangement could potentially incur", MPs said.
Following the Norwegian and Swiss trading models would "not be appropriate or advantageous" for the UK, the report said.
It added: "The Government should recognise the probability of no mutual interest deal being concluded within the two-year notice period."
The UK would move to standard world trading requirements and "would then need to decide which of the 6,987 directly-applicable EU Regulations would need to be replaced by UK law".
MPs also warned that remaining in the bloc would also pose significant challenges and played down some claims that Britain would lose influence in the world.
Pressing problems, such as the "apparent decline" of UK influence in driving EU foreign policy, slow and cumbersome processes, and the bloc's failure to grapple with extreme instability on its borders could turn into "major long-term risks for the UK inside the EU" if they are not dealt with, it found.
MPs also said it was important to "avoid over-stating" the extent of the UK's potential marginalisation in the transatlantic alliance.
Fresh on the back of the visit by US President Barack Obama to London, the committee insisted that, as Britain meets the Nato and UN foreign aid spending targets, the UK is "likely to remain an important player in world affairs and a key strategic partner for the US".
Counter-terrorism co-operation would "undoubtedly continue even after leaving the EU" and the "five eyes" intelligence-sharing network with the USA, Canada, Australia and New Zealand is unlikely to be affected.
MPs also warned that potential expansion of the EU to a bloc with 33 or more states that have large economic and political disparities "may exacerbate existing fissures within the EU".
"Without appropriate arrangements for controlling migration from new states, moreover - especially from Turkey, should it eventually join the EU - enlargement could put great strain on the resources of the existing member states," the reports states.
Committee chairman Crispin Blunt said: "The referendum offers the British people a once-in-a-generation opportunity to chart a course for the UK's role in the world. Voters should consider not only the short-term consequences of staying or leaving but the long-term opportunities and challenges."