By Elizabeth Piper

LONDON (Reuters) - A group campaigning for Britain to leave the European Union urged voters on Monday to back a British exit from the EU, or "Brexit", to prevent what it called "an Orlando-style atrocity", a message condemned by rival campaigners.

Posted on the @LeaveEUOfficial Twitter account run by Leave.EU, a day after 50 people were killed in the Florida shootings, the poster-style advertisement said: "Islamist extremism is a real threat to our way of life. Act now before we see an Orlando-style atrocity here before too long".

It accompanied a Tweet saying: "The free movement of Kalashnikovs in Europe helps terrorists. Vote for greater security on June 23. Vote #Leave."

The @LeaveEUOfficial Twitter account is linked to the Leave.EU's website. The poster disappeared from the Twitter feed about an hour after it appeared.

Several calls to representatives of Leave.EU, founded by insurance tycoon and supporter of the anti-EU UK Independence Party Arron Banks, went unanswered. An email asking for comment and to clarify that the campaign had posted the advertisement also went unanswered.

The Orlando gunman, Omar Mateen, a New York-born Florida resident and U.S. citizen who was the son of Afghan immigrants, was shot and killed by police who stormed the club early Sunday morning after a three-hour siege.

The official "Vote Leave" campaign, which has distanced itself from Leave.EU, disassociated itself from the poster.

Former London mayor, Boris Johnson, one of the leaders of Vote Leave, told reporters his campaign had nothing to do with the Leave.EU message and it was "very, very important" not to make political capital out of the shooting.

Both sides have stepped up campaigning before a June 23 referendum on Britain's EU membership, keen to break a deadlock in opinion polls which suggest that voters are all but evenly split. With less than two weeks to go, many lawmakers and voters have criticized the often bitter campaign.

The main opposition Labour Party, which supports Britain remaining in the European Union and some lawmakers from the ruling Conservative Party, which are deeply divided over the vote, condemned the poster.

"This is a shameful and cowardly poster," said Hilary Benn, Labour's foreign affairs spokesman.

"It takes courage to stand by your principles and your friends when they are attacked, whether in Orlando, Paris or Brussels," he said in a statement, referring to Islamist attacks on the two European capitals which killed more than 150 people.

"Doing so makes us stronger and shows our resolve to stand up for our values and our way of life in the face of those who hold both in such murderous contempt."

"This is really shameful," Nicky Morgan, the Conservative education minister, posted on Twitter.

(Additional reporting by Kylie MacLellan and William Schomberg, editing by Mark John)