LONDON (AFP) - A group campaigning for Britain to leave the EU briefly posted a tweet on Monday (June 13) warning that EU membership could lead to an "Orlando-style atrocity", prompting outrage from "Remain" supporters.

"Islamist extremism is a real threat to our way of life. Act now before we see an Orlando-style atrocity here before too long," read the slogan on a picture featuring what appeared to be Islamic State fighters.

The tweet on the @LeaveEUOfficial account said: "The free movement of Kalashnikovs in Europe helps terrorists. Vote for greater security on June 23. Vote Leave".

The tweet was taken down around an hour later.

Forty-nine people were killed Sunday when gunman Omar Mateen burst into a gay nightclub in Orlando, Florida and opened fire. Mateen later died in a shootout with police.

In the wake of the recent Paris and Brussels attacks, anti-EU campaigners in Britain also issued warnings about the free movement of arms and fighters in the bloc.

Their opponents say that Britain is not part of the EU's Schengen free movement zone and therefore has the border checks to prevent such activity.

Education minister Nicky Morgan, who supports Prime Minister David Cameron on staying in the EU, reacted on Twitter saying: "This is really shameful".

Hilary Benn, foreign affairs spokesman for the opposition Labour party which also supports "Remain", called the tweet "shameful and cowardly".

"Our best protection against terror is standing together, whether in Orlando, Paris or Brussels," he tweeted.

Ten days before the referendum, opinion polls indicate that the "Leave" and "Remain" campaigns are neck and neck.