The Leave.EU campaign has suggested Britons should vote to quit the EU in next week's referendum in order to stop an "Orlando-style atrocity" happening in the UK.
The tweet, sent from the campaign's official account, appears to feature a picture of armed ISIS supporters and suggests a vote to leave the EU in next week's referendum would stop similar attacks in the UK.
"The free movement of Kalashnikovs in Europe helps terrorists," the campaign said. Leave.EU is one of the two main anti-EU campaign organisations and is closely associated with UKIP.
Pro-EU campaigners attacked the decision to link the Orlando atrocity, in which 49 people were killed in a Florida gay club, to the impending vote on Britain's membership of the EU.
Shameful and cowardly. Our best protection against terror is standing together whether in Orlando, Paris or Brussels https://t.co/Cp0LOqGXxe
You use a massacre of innocent people to say this. Desperate, disgusting stuff. @BorisJohnson @GiselaStuart https://t.co/Ku2txBsQE9
Even pro-Brexit campaigner Tim Montgomerie said the tweet had echoes of the Republican US presidential nominee, Donald Trump.
All the tact and sensitivity of @realDonaldTrump https://t.co/QeBMcBYM9o
An hour later the tweet was deleted. Leave.EU could not be reached for comment on why it chose to draw the connection between the shooting and Brexit.