Cabinet ministers secretly agreed that Britain should put security on the table in Brexit talks, despite official Government denials, it is reported.
There was "panic" from EU leaders about the impact the UK's exit would have on security in the face of "increased Russian aggression", according to The Sunday Telegraph.
The paper claims leaked minutes of a Brexit Cabinet committee meeting on 7 March show ministers identified the UK's "very strong hand" on defence as a key advantage in negotiations.
The disclosure comes after Boris Johnson attempted to play down a row triggered by Theresa May in her Article 50 letter to the European Council president, Donald Tusk, by claiming Britain's support for EU security was "unconditional".
In her letter, which sparked a furious backlash in the EU, the Prime Minister said failure to reach a Brexit deal on trade "would mean our co-operation in the fight against crime and terrorism would be weakened".
But in an interview with France's Le Figaro, asked whether Britain was trying to use security cooperation as a bargaining chip to secure an EU trade deal, Mr Johnson said: "No, not at all."
He added: "We consider the historic contribution of the UK to the security and stability of Europe as unconditional.
"We will maintain this contribution, which benefits all of Europe and the world. It's in our interest and in the interest of others, and we hope this will be one of the planks of our deep and special partnership (with the EU)."
But according to The Sunday Telegraph, ministers at the 7 March meeting said security would be a "defining" issue for the EU and that Britain should not "underplay" its hand as it seeks to secure a favourable free trade deal.
The paper claims Cabinet ministers Sir Michael Fallon, David Davis, Philip Hammond and Boris Johnson are all understood to have spoken up about the importance of British security to the EU ahead of talks.
A source familiar with the discussions told the paper: "While there were nuances, I think the absolute view around the table was we are in a very strong position and the Europeans know it.
"We go into these negotiations with security and defence being a big thing in our corner."
A Government source has declined to comment on the leak, but said that Britain wants "a deep and special partnership with the EU covering all aspects including security".
The PM faced allegations of "blackmail" and "threats" by EU figures after her Article 50 Brexit letter mentioned security 11 times across six pages.
On the same day Amber Rudd, the Home Secretary, said in a Sky News interview: "If we left Europol, then we would take our information with us."
Number 10 has insisted no "threat" was intended and that the UK Government was not explicitly linking the country's help on security with a trade deal.
However, The Sunday Telegraph claims to have obtained an official account of the 7 March meeting, at which a paper on Britain's defence and security drawn up by the Ministry of Defence and the Foreign Office is said to have been submitted for discussion.
According to the paper, it both spelled out the country's existing EU security links and reported back what EU leaders were thinking on the topic, including rising concerns over Russian aggression.
Mr Johnson, the Foreign Secretary, led discussions by saying that Britain had a "strong hand" at the negotiating table when it came to security, the paper reports.
He stressed that "we should not be a demandeur" in negotiations when it came to security - the French word for someone who makes demands - but be aware of the tactical advantage.
Sir Michael, the Defence Secretary, is understood to have said: "Insecurity in Europe is at its highest since the Cold War. There is increased Russian aggression.
"The EU needs our capabilities," he is recorded as adding, before saying that the UK had "high cards" to play at the negotiating table on security.
Mr Davis, the Brexit Secretary, is noted as saying: "I agree that Britain has high cards and a very strong hand [on security].
"The UK is in a strong position and there is a degree of panic within Europe over defence policy and cooperation." He is said to have added that the UK should not "underplay" its hand.
Mr Hammond, the Chancellor, said that the EU would not offer an economic relationship which deterred Britain from co-operating on defence. He added the UK should make that "clear" during talks.
The Prime Minister is said to have attended the meeting and summed up at the end. She is understood to have said that defence and security were a "defining issue for the EU" and that the UK is in a "strong position".
Those familiar with the talks said there was no mention of "threatening" the EU but an acceptance the UK could use its strong position during negotiations.
A Government source told The Sunday Telegraph: "We do not comment on Cabinet Committee leaks, however we have been clear that we want a deep and special partnership with the EU covering all aspects including security.
"Our position is simply a statement of fact - if we leave with no agreement in place, the arrangements we currently have will lapse. We are not bargaining security with anyone - we want to maintain the degree of cooperation on these matters that we have currently."