Bookmakers' odds have shifted sharply toward Britain voting to remain in the European Union in the June referendum, a move in sentiment also reflected by the pound rising Monday to a near six-week high against the euro.

Both developments followed a high-profile intervention by U.S. President Barack Obama in the EU debate, but supporters of leaving the EU warned the "In" camp not to celebrate too early.

Obama said Britain would find itself "in the back of the queue" for a trade deal with the United States if it voted to quit the EU on June 23, and said it would be safer, more prosperous and more influential if it stayed in.

Following his unexpectedly forceful intervention, the implied probability of a vote to remain in the 28-nation bloc rose sharply to about 75 percent, according to live odds from bookmaker Betfair.

Ladbrokes' betting barometer, also based on live odds, indicated a 73 percent chance of an "In" vote.

Sterling hit a six-week high against the euro of 77.52 in early Asian trade, with traders saying Obama's intervention was helping sentiment as it underlined the weight of argument from global and financial leaders in favor of the "Remain" camp.

The EU issue has split the ruling Conservative Party, with Prime Minister David Cameron leading the campaign to stay in while six of his cabinet ministers and a large proportion of his party's lawmakers are openly campaigning for Brexit, as leaving is known.

The "Leave" campaign fought back Monday, with figurehead Boris Johnson, the departing Conservative mayor of London, seeking to undermine the arguments made by Obama.

"For us to be bullied in this way ... for people to say we are not going to be able to cope on our own is completely ridiculous," he told ITN television.

"Saying we have to go to the back of the queue seems to me ridiculous when actually the real reason we haven't been able to do a pretrade deal with the U.S. is because we're part of the EU," he added.

In an opinion column in the Daily Telegraph newspaper, Johnson accused the "Remain" camp of "crowing too soon."

Another prominent "Leave" campaigner, Justice Secretary Michael Gove, wrote an article in the Times newspaper portraying the European Union as profoundly dysfunctional, undemocratic and misguided in its response to Europe's biggest challenges.

He said the next wave of EU expansion, which could include Turkey and Albania, would create a migration "free-for-all" and place "unquantifiable strain" on Britain's public services as millions more people gained the right to move into the country.

But Home Secretary Theresa May, an interior minister who is in the "Remain" camp, took on the immigration issue, which polls suggest is salient with voters.

May, who has adopted a hard line on immigration in speeches, is thought to be a reluctant "In" campaigner who is following her head rather than her heart. Many had expected her to come out in favor of "Out" until she stated her position.

She said in a television interview Sunday that Britain's EU membership made it more difficult, though not impossible, to control immigration. The comment was seized upon by the "Leave" campaign.