The UK Independence Party leader Nigel Farage has admitted that that Vote Leave camp in the British referendum debate was at risk of losing the argument by relying too heavily on Tory cabinet ministers "instead of allowing alternative voices to be heard."
"It appears that the Leave campaign is just the Conservative cabinet ministers," he said.
"I think to win this referendum you've got to see trade union voices, you've got to see Labour voices, you've got to see Ukip voices - we've got to be appealing to a broader group of people than just the Tory electorate."
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His comments come after US President Barrack Obama, speaking in London at the weekend, served a big blow to the Leave campaign saying the UK will be in the "back of the queue" in negotiating a trade deal with the US, if it were to leave the EU.
In a direct appeal to British voters, he said that the EU magnifies British foreign policy, while Britain helps to keep the EU from becoming introverted.
Elsewhere, Hillary Clinton added her voice on Saturday to those from across the Atlantic urging Brits to vote to stay in the EU in a June 23 referendum.
"Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united," Clinton's senior policy adviser, Jake Sullivan, said in a statement to UK paper the Observer.
"She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU."
In the wake of the American interventions, Farage, an MEP, said Leave campaigners should focus more on the issue of immigration, instead of the economics of the EU. "If we debate economics and trade, we can go round in circles for weeks and the public will be none the wiser," he said.
It appeared that peace had broken out on the Leave side last week when Farage, whose Grassroots Out campaign failed in its bid to be declared the official Leave campaign, shared a platform with the pro-Brexit minister Chris Grayling.
But Farage said it was time for a fresh start after a shaky week in which Vote Leave's campaign director Dominic Cummings endured a tough grilling from MPs and London Mayor Boris Johnson was slapped down by Obama.
On Monday, it also emerged that Gisela Stuart, herself a former MEP and now the co-chair of Vote Leave, had written to the UK home secretary Teresa May urging her to exclude the right-wing French MEP Marine Le Pen, who plans to visit Britain to make the case for Brexit.
Amid fears that the arrival of the leader of the Front National to back Brexit would damn Vote Leave by association, Stuart said Le Pen had made "many divisive and inflammatory comments, including comparing Muslims praying in the street to the Nazi occupation of France."
Dominic Raab, the pro-leave junior minister, also said he hoped Le Pen would stay away.
Meanwhile, former EU trade Commissioner Mario Monti says he is worried about the break-up of Europe.
Monti, a former Prime Minister in Italy, said he fears that a combination of morally corrupt national politics, structural holes in the Brussels machine, and external crises may trigger the collapse of the EU.
"The EU is going through a crisis which leads me and others for the first time to consider whether we are not heading towards disintegration," he said.