Michelle Harper, 25, and her partner Brian Slusarek, 28, have lived together in the UK since January 2015 - but now Brian has had his application for a permanent stay rejected and the pair now face being split apart.
Immigration chiefs say the unmarried pair can't prove they've lived together for 24 months - the threshold needed to qualify for leave to remain - after rejecting the evidence they submitted to show they also cohabited in Canada.
Brian, who has no criminal convictions, is currently on a £230 two-year Tier 5 (Youth Mobility Scheme) visa, which expires on January 6 next year, and has worked as a business manager in the UK.
They lived together in Brian's family home in Canada in 2014, but they have no paperwork from Canada and the Home Office only accepted their time living together in the UK - one year and nine months at the time Brian applied.
Michelle told the Daily Mirror: "We're both devastated really. We're a genuine, honest couple and we've done everything by the rules.
"Brian is the most hardworking person I've ever met, and I know that anyone who knows him here would say the same.
"Besides all of that, he has done nothing but love, support and provide for me over the past three years and they have hands down been the best three years of my life.
"Even more, Brian has provided for this country. Last year he paid over £5,000 in taxes from wages alone, not to mention additional contributions to National Insurance, council tax and all the rest of it.
"Brian is from a country in the Commonwealth . We share the same Queen, the same language, and same cultural values. He has no criminal convictions, no children and no previous marriages.
"Yet besides all of this, the government have refused his visa to remain in the UK and he must return to Canada on January 6, no exceptions."
The couple met in Cairns, Australia, in November 2013 and travelled together until the following April, when Brian returned to Canada.
In June 2014 Michelle moved in with Brian at his family home in Barrie, Ontario, and stayed there until her three-month visitor visa expired.
Brian then moved to the UK in January 2015 and the couple originally rented a house in Stoke before moving to Sheffield this summer.
Michelle said: "We thought it would be pretty straightforward for Brian to stay here. We paid £811 each to apply in September and they asked us to prove we'd been together for two years.
"Unfortunately when we lived in Canada, I didn't open a bank account or have any form of paperwork to prove we were together.
"We've been to see an immigration lawyer but basically the advice was that Brian will have to leave. His employers have said they will keep his position open if he returns.
"They said basically we should have just got married in the first place. But if we get married now they will reject it as a sham.
"Because I'm studying and not working it makes things even more difficult. You have to have a certain amount of savings. To meet the requirements we'd have to have £62,500.
"They're basically putting a price on love, which is ridiculous. Now I just wish we'd got married, but we wanted to do things at our own pace and not rush into it.
"The rejection letter came last week and basically just said we don't meet the relationship requirements, we're three months short.
"It came as a shock. You ask yourself all these questions: why won't they let him stay? It made me really upset. We feel betrayed in a way because he's such a hard-working person.
"He's trying to be strong for me and he's a very positive person, but it's hard to talk about it. He's looked at flights to go back to Canada in January but not booked anything."
Michelle, originally from Stoke-on-Trent, Staffordshire, is studying for a masters degree in clinical communication studies at the University of Sheffield , while Brian works for a car manufacturer in Mansfield, Nottinghamshire.
Brian said: "I'm definitely frustrated by it. I thought it was quite an easy process, but it's all come down to the fact we're missing a bit of paper from Canada.
"I thought it was going to be a done deal. I signed a one-year lease for the house in August and I wouldn't have done that if I thought I'd be leaving.
"In hindsight obviously I wish we'd got a bank account or something that showed we're together. We've shown them photos and evidence from our families but it's not enough.
"It's hard to stomach. There's not much point appealing as I have nothing new to submit so it'd be a waste of £800.
"We've just got to take each day as it comes and assess our options. We've spoken about getting married and starting a family and that's still the plan."
A Home Office spokesman said: "All applications are considered in line with the immigration rules and accompanying guidance.
"The guidance is clear that unmarried partners require evidence of cohabitation for two years and, despite being given the opportunity to provide evidence from before he came to the UK, Mr Slusarek could not.
"It is open to Mr Slusarek to reapply before his current visa expires, at which time he should have accumulated the correct evidence."