US president Barack Obama has been warned against intervening in the EU debate by pro-Brexit MPs ahead of his visit to Britain next month.
In an open letter the cross-party group said it would be an "unfortunate milestone" at the end of Mr Obama's time in office should he enter the debate with even a "passive diplomatic recommendation".
It comes after Boris Johnson labelled Mr Obama's suggestion that Britain would be weaker on the world stage outside the 28-member bloc as "outrageous" and "wholly fallacious".
The letter's signatories said that, while they understood the president was obliged to speak in the interests of the United States, any intervention risked being undemocratic.
Labour backbencher Kate Hoey said the letter aims to caution Mr Obama that "feelings will run high" if he expresses a side in the debate.
She said: "We have chosen to respectfully request he recognises matters of sovereignty are best left to the citizens directly affected.
"We would certainly never think of visiting the United States and telling the US public how to vote in an election or the amendment of their constitution."
The letter, which is the initiative of Leave.EU and the Grassroots Out Brexit campaigns, carries the signatures of Ukip's Nigel Farage, Labour MP Kelvin Hopkins, Tories Tom Pursglove and Peter Bone, and the DUP's Sammy Wilson.
Copies are due to appear in several political journals delivered to the White House and members of the Congress and Senate, as well as political offices on Capitol Hill.
Tory backbencher Mr Bone said that however Mr Obama thought a Leave vote on June 23 may affect the US, it would be better for the "special relationship" if he "kept his counsel to himself".
The group said the referendum will be a "vote of profound consequence" and it is "imperative that the question of exiting the European Union is not one answered by foreign politicians or outside interests, but rather by the British people who must ultimately live with change or the status quo".
They told the president that Britain could "thrive" outside the EU by being free to operate without tariffs and trade restrictions while having sovereignty over its affairs and control over its borders.
The vote "puts the matter of national destiny in the hands of the citizenship", they said.
The letter added: "While it is understandable that a sitting US president feels the obligation to speak in the interest of the United States, it must be advised that even a passive diplomatic recommendation in the matter of our national decision will receive the opposite of the intended effect.
"The referendum vote is an act of democracy in its most direct form, and the question of whether or not to leave the EU is a rare political topic that is not owned by any one political party. This is a chance for the British people to choose the path of their country. Interfering in our debate over national sovereignty would be an unfortunate milestone at the end of your term as president.
"As fellow elected representatives, we would therefore ask that you abstain from any intended advocacy in the matter of the approaching referendum, and allow democracy to take its course."