The Home Office set out criteria for non-EU players to come to English football, demanding that they must have played in a certain percentage of their national team's matches over the two years prior to their application. The precise percentage depends on their country's FIFA ranking.
If this rule was also applied to players from EU countries, more than 100 Premier League players would have failed to gain a work permit.
Extend this to the top two divisions in England and Scotland, and this rises to something like 400.
Among them would be Manchester United striker Anthony Martial, West Ham's Dimitri Payet and Leicester's N'Golo Kante, all of whom are relatively new to the France side.
If Britain had never been in the EU and these rules had always been in place, we'd never have seen Cristiano Ronaldo or Thierry Henry come to the Premier League so young.
The first thing to say is that Premier League stars from EU nations aren't going to be kicked out following Brexit. Any new rules won't be applied retrospectively.
However, in the long term, if regulations were imposed, we could see gems like Payet, Kante and Martial move somewhere other than the Premier League and that would certainly weaken its status as one of the world's best leagues.
What is likely to happen, following the no doubt long and complex negotiations at a national level, is that players from EU countries will retain a 'favoured' status, making signing them still easier than players from outside the EU.
Daniel Geey, of law firm Sheridans, told Sportsmail: 'The question is whether European players will have a harder time joining Premier League clubs, whether there is now a disincentive, whether things become difficult.
'It could be that clubs look increasingly to the UK markets and that the value of UK players - who wouldn't need visas - rises.
'But is could also be that it actually reduces the price of European players because of all the bureaucracy involved.'
FOOTBALL: YOUNG PLAYERS
In addition to this, Premier League clubs are now in danger of missing out on talented teenage players from Europe.
Article 19 of the FIFA Regulations, concerning the Status and Transfer of Players, permits the 'transfers of minors between the age of 16 and 18 within the EU or EEA [European Economic Area].'
It means with Britain outside the EEA that leading Premier League academies such as Chelsea and Manchester City, who like to import up-and-coming players from European clubs, would find it harder to do so.
One example of a player who'd no longer be able to come to the Premier League is Arsenal's Hector Bellerin, who joined the club aged 16.
FOOTBALL: TRANSFER FEES
The falling value of the pound against the Euro could have a knock-on effect on transfer fees, at least in the short term.
Premier League clubs do now face a hike in prices, with millions potentially added to the price of players overnight when buying from Europe.
Though the Premier League, thanks to its lucrative television deals, retains immense spending power, some clubs may think twice if costs of elite players become prohibitive.
For example, the 40m Euro offer made by West Ham for Marseille's Michy Batshuayi has already risen from £31m last week to £34m now before of the falling pound against the Euro.
Geey adds: 'The Premier League is always going to be a net importer. It depends now on whether the transfer fee is paid in pounds or whether it is paid in Euros.
'Likewise with wages for European players, they could be paid as a guaranteed mark in Euros and then converted into pounds afterwards.'
FOOTBALL: PLAYER QUOTAS
Brexit poses a headache for certain Spanish clubs, with La Liga rules allowing a maximum of three non-EU players in their squad.
At Real Madrid, with Welshman Gareth Bale now classed as a non-EU player as well as James Rodriguez of Colombia and Brazilian duo Danilo and Casemiro, they will exceed this quota.
With the Bernabeu club reportedly set to offer Bale, twice a Champions League winner during his three years there, a 'contract for life', one of the others will have to go.
RUGBY AND CRICKET: KOLPAK AGREEMENT
Away from football, we could see the end of the Cotonou Agreement and the Kolpak Ruling in 2003 that allows players from Africa, Caribbean and the Pacific Group of States (ACP) enjoy the same rights as players from the EU.
This ruling has enabled many players from South Africa, the Caribbean and the Pacific Islands have come over to the UK to play in our domestic leagues.
Some have then gone on to play for the England teams - South African-born players such as Kevin Pietersen and Andrew Strauss in cricket and Samoa-born Manu Tuilagi in rugby, for example.
Brexit will render the Kolpak agreement void in the UK, complicating the signing of such players and potentially weakening the cosmopolitan nature of the leagues.
The Kolpak agreement is named after a former Slovak handball goalkeeper, Marek Kolpak, who won a case against the German Handball Federation at the European Court of Justice in 2003 to allow freedom of movement.
At present, there are around 70 cricketers who play for counties in England and Wales employed under this agreement.
TENNIS: WIMBLEDON PRIZE MONEY
Wimbledon gets underway next week but Brexit is already having a significant effect on the prize money on offer to the players.
Because the culmulative £28.1m prize fund is given in sterling, this would be worth less to an overseas winner if the pound continues to fall against foreign currencies.
Both the men's and women's singles champions will earn £2m this year. This is an increase of 6.4 per cent on 2015, but the rise is almost certain to be negated by the falling value of the pound.
GOLF:THE RYDER CUP
Given the many talented golfer that come from the British Isles, will Brexit have implications for the European Ryder Cup team in the future.
The next Ryder Cup is set to take place at Hazeltine in September, but the European Tour insist there will be no change to tradition.
A spokesperson for The European Tour said: 'The criteria for being European in Ryder Cup terms is a geographical one (ie from countries who make up the Continent of Europe) not a political or economic one (ie from countries who make up the EU). Therefore the result of the UK referendum has no bearing in Ryder Cup qualification terms.
'In terms of the flag flown to represent the European Ryder Cup team, we consider that the blue and gold flag of Europe represents the continent of Europe and, as a broad symbol of Europe as a whole, we therefore plan to continue to use it.'
