"I am delighted at the Sun's initiative, and I believe it will have a significant impact. It will sway public opinion – and also the Sun would not have done this unless they thought they were on the winning side," Helmer said.
He added that the tabloid’s initiative could motivate other local media to clearly define their stance on the issue.
"I wouldn't be surprised if other papers came out – indeed some have already made it clear. [The] Guardian, Indi [The Independent] and FT [The Financial Times] are IN, while the Telegraph is OUT," the lawmaker said.
UK citizens are set to vote on June 23 in a referendum on the country's EU membership, after British Prime Minister David Cameron and the leaders of the 27 other EU member states reached a deal in February to grant the United Kingdom a special status within the bloc.