A fresh top-level international intervention in the Brexit battle is on the cards as Japan's prime minister Shinzo Abe heads to Downing Street for talks with David Cameron.
Mr Abe has already made it clear he backs Mr Cameron's Remain stance as Tokyo wants Britain to stay a "strong voice" within the EU.
Though lacking the heavy weight, high profile of US president Barack Obama, another foreign foray into the debate is likely to provoke a backlash from the Leave camp.
Visiting London ahead of hosting a summit of the G7 leaders of the world's richest industrialised democracies later this month, Mr Abe is set to be drawn into the ferocious political fight over Britain's future status in Europe ahead of the June 23 referendum.
Mr Obama's unexpectedly blunt warning that Britain would be forced to the "back of queue" when it came to trade deals if it cuts ties with Brussels enraged Brexiteers, but was seen as a boost by the Remain campaign.
The Japanese leader will be keen to avoid the vicious personalised criticism directed at Mr Obama after his intervention, but his government has made it known that it sees EU membership as a major attraction for the country's businesses to invest in the UK.
The meeting comes after Mr Cameron was thrown on to the defensive over the deal he negotiated with Brussels for a "special status" for Britain in the EU as he was subjected to sustained questioning at the Commons Liaison Committee.
Mr Cameron insisted he secured "fundamental" reforms, after he was accused of offering a "false prospectus" to voters.
The Prime Minister also urged voters not to be swayed by concerns about Turkey possibly joining the 28-nation bloc.
He was speaking just hours after the European Commission gave its conditional backing for Turks to get visa-free travel inside Europe's passport-free Schengen area, which does not include the UK.
"I will be absolutely frank with you, I don't think the accession of Turkey to the European Union is remotely on the cards. I don't think it will happen for decades," Mr Cameron said.
Veteran Eurosceptic Sir William Cash accused the PM of "cheating" referendum voters, because the outcome of the renegotiation did not allow him to guarantee change in the EU's treaties before they cast their votes.
But Mr Cameron insisted the deal reached in Brussels in February delivered "fundamental" reform to remove Britain from the commitment to "ever-closer union", guarantee equity of treatment for the pound and address concerns over migrant welfare.
Mr Cameron said he was not "over-emphasising" the significance of the renegotiation deal, and stressed the referendum was not a judgment on his package, but on the wider issue of whether the UK should remain in the EU.
He insisted that he would stay on as Prime Minister to oversee withdrawal negotiations if Britain voted to leave.
Labour committee member Frank Field challenged him, saying: "So you are seriously thinking that if the vote goes against you, you can remain as Prime Minister?" Mr Cameron replied: "Yes."
"And if the country thinks otherwise, you just ignore them?" asked Mr Field. "They would be voting against you and your recommendation."
Mr Cameron denied that the Remain camp was running a "Project Fear" campaign to scare voters in to opting for EU membership with threats of the damage departure could do to the economy and jobs.
The PM also admitted that the mechanism banning migrants from claiming full benefits for four years was "quite arcane", but would work.