Britain must be part of a European army after Brexit so Europe can protect itself during crises like migration, a French senator has said.

Europe is unable to defend itself without the help of the US and needs to develop a credible military response to emerging threats, according to Hélène Conway-Mouret.

The Secretary of the Committee on Foreign Affairs, Defense and the Armed Forces, says Europe lacks a robust mechanism to meet security challenges below the threshold of Nato engagement, such as migration and terrorism.

In a report jointly written with fellow committee member Senator Ronan Le Gleut, the French parliamentarians say Europe has to “rouse itself from its inaction as a geopolitical entity” if it is not to “fall into a kind of vassalage”.

Britain will not “drift away anywhere” they say, and Brexit offers a “historic opportunity for Europe to strengthen its defence”.

Senator Conway-Mouret told me the establishment of a defence and security treaty with the UK after Brexit should be a priority for the EU.

“It will be Britain as one entity dealing with us collectively, that’s the only change,” she said.

“Apart from the UK and France, none of the [EU] countries have made a choice to invest in their armies. Having the cover of Nato, reassuring everybody, no investment was done.

“With you leaving, France is the only army left in the [European] Union with the whole range of capabilities.”

She said Europe lacked the ability to provide a coordinated response to cross-border terrorist attacks and migration.

“What does Nato do tomorrow if we have waves of terrorist attacks? Nato hasn’t been conceived to deal with that. Europe reacts slowly and when suddenly you have to deal with thousands of people, what do you do?” she asked.

Donald Trump was right to accuse France and other European Nato members of not meeting their pledge to spend 2 per cent of GDP on defence, Senator Conway-Mouret said.

“Individually, even if we increase our budgets we would not be capable, unless we have an overall strategic view of the threats and how we can cope with them, to be able to invest sufficiently.

She also warned that the US administration under Donald Trump had continued Barack Obama’s strategic policy of a pivot to Asia.

“The Americans are showing that whilst we may still be friends, we are not a priority.”

“Donald Trump is forcing us now, in a very brutal way. He’s saying: ‘well, now you need to fend for yourselves’. We’ve been shaken very brutally into waking up.

Senator Conway-Mouret rejects the idea of a standing European army and blames “linguistic and semantic differences” for creating the impression the EU was trying to raise its own army.

She compares the concept of such a force with the relationship member states have with Nato: no standing troops, but established mechanisms to commit military capabilities as the situation requires.

“We refute…the concept of a European army,” she said.

“It is absolutely wrong. it’s a concept that is far fetched and no use.

“Nobody wants it,” she added. “We went round the Poles and the Finns, the north and the east. This is not what European defence is about.”

She agreed with General Pierre de Villiers, former Chief of Staff of the French armed forces, that the idea of a European army should never happen as it is “a dream which could turn out to be a nightmare”.

The Senators argue that Europe gave up on providing for its own defence in the second half of the twentieth century, due to the “political, material and moral weakening” caused by the two World Wars, and the superpower rivalry played out on the continent thereafter.

Since the Second World War the US has spent $35.8 billion on the defence of Europe, they say.

She added that after Brexit Britain should continue to have access to the European Defence Fund, valued at 13 billion euros over the years 2021-2027, although there are attempts to drastically reduce that figure.