European Union (EU) security strategies for dealing with "unstable and dangerous" neighbours have "glaring weaknesses", peers have warned.
Brussels was "unprepared" for the increased terror threat and the refugee crisis, according to a House of Lords report.
It claimed the 28-member bloc's approach to Turkey is in "disarray" and called for an urgent policy review.
Peers criticised the EU's response to Russian aggression and called on it to come up with a "credible" response.
The EU sub-committee on external affairs also warned Britain's international influence would be "significantly limited" if it leaves the EU.
Committee chairman Lord Tugendhat said: "We believe the EU faces a further test should the UK decide to leave the EU.
"If this were to happen, the UK's international reach would be significantly limited because it would not have access to Commission instruments. In turn, the EU's foreign policy would carry less weight.
"Although there have been some EU foreign policy successes, notably the nuclear deal with Iran and maintaining effective sanctions on Russia, we believe that there is much room for improvement on how the EU's foreign and security policy operates."
Confusion over Turkey's future as an EU state must be resolved because of the state's role as a "critical buffer state" in the refugee crisis and the fight against Islamic State, peers said.
They criticised the EU, as well as Nato, for its "inadequate" deterrence in the Baltic States and Black Sea, and said it was "unclear" if Russian military action would be met with a "forceful" response by European states.
Peers claimed the "wariness" of EU countries to use legitimate and proportionate military force has undermined the the bloc's status as a "foreign policy actor".
They heard evidence in favour of increased military co-operation across the EU, with suggestions put forward to the committee for more "joint military procurement".
The committee pointed to differences in defence policies and spending levels as a barrier to increased reliance between countries.
Lord Tugendhat added: "The threats to stability in the EU neighbourhood have changed dramatically since the EU last agreed a security strategy, over 12 years ago.
"We see the most direct challenges to the EU's security and stability as originating in the immediate European neighbourhood, where the new strategy urgently needs to focus its attention.
"It is clear to the committee that there are glaring weaknesses in the EU's current approach to a number of countries.
"A reassessment of the EU-Turkey relationship is imperative, particularly given the increasing strategic importance of Turkey in light of the war in Syria and the refugee and migration crisis.
"We welcome dialogue with Russia on areas of shared interest, such as the Middle East, while maintaining sanctions in relation to its breaches of international law. The Russian bombing operations in Syria over the weekend came after the completion of the report and must be condemned."
Alan Johnson, chairman of Labour In for Britain, said there was "overwhelming" evidence that leaving the EU would lead to a loss in international influence for the country and the union.
The MP said: "Britain's voice around the world is massively enhanced by our membership of the European Union which brings us jobs, investment and protections for working people, and as this report shows, it makes Britain stronger and safer.
"Whether responding to Putin's Russia, negotiating the deal that has halted Iran's nuclear programme, combating instability in the Middle East or negotiating trade deals, it is clear that Britain is better off in Europe.
"The evidence is overwhelming - leaving the EU would lead to a loss of international influence for Britain and for Europe. There is nothing patriotic about diminishing Britain on the world stage."