"The only people who should play for England are English people,'' he said after training at St George's Park in preparation for Friday's World Cup qualifier with Montenegro.
"If you live in England for five years it doesn't make you English. You shouldn't play. It doesn't mean you can play for a country. If I went to Spain and lived there for five years I'm not going to play for Spain.''
Januzaj's superb brace for United on his full debut against Sunderland at the weekend, and a string of glowing reports about this well-balanced teenaged attacker's gifts in training and on pre-season tour, have excited an FA determined to be more aggressive in stocking England's shallow player pools. But Wilshere is right. The FA needs to think again.
In seeking to reprise the Child Catcher role from Chitty Chitty Bang Bang, the FA reveals cynical traits in considering pinching other nations' prospects.
Januzaj has yet to declare an interest in which country he will represent but the FA risks embarrassment simply in signalling its intent to play pass-the-passport.
It should be focusing its energies on nourishing grass-roots across the country, not transmitting a negative message towards the properly home-grown by raiding nations' talent factories.
After Wilshere spoke out so robustly against the idea of Januzaj at St George's Park, the Under-21s coach Gareth Southgate also addressed the issue in the Sir Bobby Robson room.
During his 18 months as FA's head of elite development, Southgate campaigned across the country to persuade counties and leagues to introduce small-sided football and nurture more skilful players who might one day represent England.
He now works with the age-group sides, also coaching the Under-21s, to ensure an improved flow of more rounded players for the senior side.
Would exploiting a loophole and parachuting a player in send the wrong message to those involved in player development?
"I am torn with it,'' the former England captain said. "You will have a split jury. Personally, I wouldn't have wanted to play for anyone else, even if there had been a loophole.''
Yet Southgate sympathised with the FA stance.
"Had he [already] played for somebody else that would be more of an issue for me. It's slightly different to some of the cricketers.
"We seem to have embraced the England cricket team [some of them raised in South Africa] that have won the Ashes.
"We have lots of boys in our [Under-21] squad who weren't born here but whose families have fled here, and some wonderful stories throughout. Saido Berahino, Wilf Zaha and Nat Chalobah are all proud to play for England.''
Unlike Januzaj, who came to England aged 16, those three have been raised here, doing the majority or all of their education here.
Berahino was 10 when he escaped Burundi in search of a better life and is passionately proud of representing England all the way up to Under-21 level (although also realistically acknowledging the restrictions that opting for his "motherland" of Burundi would place on his international development).
Zaha arrived in England from the Ivory Coast aged four. Chalobah, who was born in Sierra Leone, has lived in England since he was 11.
These three have been part of the system, sporting and educational.
Januzaj is different. He is not a political or economic refugee. He came solely for career reasons because United spotted his talent and lured him from Anderlecht.
He has no loyalty to England, and it may well transpire that he has absolutely no interest in either playing for a nation where he just happens to ply his trade or wait five years before making his mark on the international scene.
"It's a really interesting philosophical debate,'' Southgate said. "Historically we would have viewed it in a certain way [not even considering the possibility of nationalising him] but the world is evolving. The world is changing, families are moving more and more, working abroad.''
That happened to the Cheltenham-born Eric Dier. The Sporting Lisbon defender's mother, the daughter of the FA's late secretary Ted Croker, moved to Portugal to work for Uefa at Euro 2004. Dier was raised there but eschewed the overtures of the Portuguese FA. He has English blood, an English affinity.
Wilshere challenged the FA further by declaring that he would prefer it if the national team coach were always English.
"I think it's better if there is an Englishman but don't get me wrong Fabio Capello did a lot for my England career. He bought me here and gave me my debut and stuck with me from a young age so he was a good manager as well."
The FA chairman, Greg Dyke, has indicated he would consider a foreign manager as Roy Hodgson's eventual successor but now knows there will be opposition from quarters of the England dressing-room.
The FA certainly knows the backlash if it continues its ill-judged flirtation with a footballer who belongs to others. The FA has a duty to English kids and to ensure the flag of St George never becomes a flag of convenience.
Lionhearts? Stars who were born abroad
Five former players born overseas who went on to represent England ...
Terry Butcher (born Singapore Dec 28 1958)
Defensive linchpin for Ipswich and Rangers and famously shed blood in England shirt. Won 77 caps.
John Barnes (born Jamaica Nov 7 1953)
Illustrious career with Watford, Liverpool and Newcastle. England high point was brilliant goal against Brazil in the Maracana Stadium. 79 caps.
Luther Blissett (born Jamaica Feb 1 1958)
Joined Watford from school, also played for AC Milan. First black player to score hat-trick for England. 14 caps.
Tony Dorigo (born Australia Dec 31 1965)
Played for Aston Villa, Chelsea and Leeds. Was part of England's 1990 World Cup squad. 15 caps.
Owen Hargreaves (born Canada Jan 20 1981) Midfielder with Bayern Munich and both Manchester clubs. Was voted England's best player at 2006 World Cup. 42 caps.
And five foreign-born players who could be future England stars ...
Saido Berahino (born Burundi Aug 4 1993)
The West Bromwich Albion striker has played at all England junior levels.
Nathaniel Chalobah (born Sierra Leone Dec 12 1994)
Chelsea midfielder has also represented England from U-16 level and up.
Wilfried Zaha (born Ivory Coast Nov 10 1992)
The Manchester United striker already has two England caps.
Adnan Januzaj (born Belgium Feb 5 1995)
The in-form United striker is eligible to play for Belgium through birth, and Albania, Turkey and Serbia through his heritage. Could be eligible for England in 2018.
Raheem Sterling (born Jamaica Dec 8 1994)
Liverpool winger has already won one England cap.