Foreigners could ultimately determine whether Britain leaves the European Union (EU), with as many as 1.3 million Commonwealth citizens eligible to vote in the upcoming referendum.
General election rules have been applied to the June 23 'Brexit' referendum, meaning that while Europeans won't be able to cast a ballot, a number of Commonwealth citizens living in the U.K. will qualify to vote.
Commonwealth citizens include those hailing from one of the 53 former member states of the bygone British Empire, including Canada, Jamaica, Kenya, New Zealand, Nigeria and Dominica.
The U.K. is set to hold the vote on Thursday, June 23. The ballot is set to ask: "Should the United Kingdom remain a member of the European Union or leave the European Union?" Voters will be asked to choose one of two options: "Remain a member of the European Union" or "Leave the European Union."
British and Irish citizens, as well as U.K. expats who have lived abroad for less than 15 years, will also be able to vote. No other EU citizens — except those from Malta, Cyprus and Ireland — will be eligible.
All voters must be 18 or older.
The long-standing voting rules haven't gone without criticism.
"It is a clear anomaly that those who are not British citizens should have a vote in the future of Britain," Lord Green, the founder of the campaign group Migration Watch UK, told CNBC by phone on Tuesday.
"Of course we are not opposed to Commonwealth citizens who are also British citizens having a vote. But if they are not yet British or have decided not to become British it is surely wrong that they should be able to."
The latest calculations by Migration Watch UK put the total number of Commonwealth citizens potentially eligible to vote in the referendum at 1.3 million. About 341,000 Irish nationals may also qualify, they told CNBC by email on Tuesday.
Neither the Electoral Commission nor the U.K.'s Office for National Statistics (ONS) were able to confirm this figure, telling CNBC that electoral registration data for general elections doesn't break down by nationality.
Similarly, the ONS did not have a total figure for Commonwealth citizens in its latest census data.
However, Electoral Commission rules stipulate that not only do Commonwealth citizens have to live in the U.K, but be granted immigration permissions, or not require them in the case that they might be dual citizens, for example.
Sophie Barrett-Brown, an immigration lawyer and senior partner with Laura Devine solicitors in London, told CNBC that there are about 70 different immigration categories through which non-citizens can obtain 'leave to remain' status -- a blanket term for resident immigration permission which is required for voter eligibility.
One of these specifically applied to Commonwealth citizens — accounting for U.K. ancestry — but otherwise they are under all the same rule as all other visa applicants, Barrett-Brown told CNBC by phone.
Voter registration for the EU referendum is open until June 7, 2016.