President Obama should not tell Britain to remain a member of the European Union unless he wants the United States to join it as well, Iain Duncan Smith has said.
In a speech in Washington DC, the former cabinet minister and leading Brexit campaigner said it was "inconceivable" that any American president would sign the US up to the deal the UK has within the EU.
Obama is due to visit London at the end of this week and is expected to intervene in the referendum debate and urge Britons to vote to Remain in the EU.
The White House's decision to take sides in the campaign has angered pro-Brexit MPs.
Duncan Smith said: "It's inconceivable that the president of the United States would be asking to do exactly the same for the USA as now appears to be the case, or might be the case, for him to advise the UK to do with regard the EU.
"On June 23, I think the British people will be advised to vote to get Britain to look a little bit more like the US and a lot less like it does at the moment with regard to the power of the EU."
He added: "I don’t quite understand why any American president would want Britain to be any other way - unless of course they want the US to join the EU too.
"Maybe that is the subtext of the speech or comments about to be made."
Boris Johnson has accused the president of “outrageous and exorbitant hypocrisy” for advising the UK to remain inside the EU when the US would never accept a similar arrangement with other countries.
Republican Senator John McCain, who lost the 2008 presidential election to Obama, has also said Britain’s membership of the EU is "vital" for the security".
The chairman of the US Senate Armed Services Committee said last month: "Having a strong British voice in NATO and the EU helps ensure we make the most of the collective power of these institutions.
"The need for a strong and united Europe is greater than ever. The United States has long benefitted from British leadership in NATO and the EU – for instance, leading sanctions against Russia, providing vital support to Afghanistan and Ukraine, and grappling with the refugee crisis fueled by Vladimir Putin,” he said.
"The United States and the United Kingdom are confronting the most diverse and complex array of crises since the end of World War II. To meet these challenges, we need a comprehensive response that makes the best use of our collective economic, political, and military power."
Ahead of his visit, Nigel Farage has described Obama as “the most anti-British president” there had ever been.
Downing Street has defended the right of foreign leaders to voice their opinion.
And Obama’s spokesman has said the president would “continue to make clear” that the United States wanted to see the UK remain inside the EU.