Passenger Lisbet Stone says she is stuck in Madrid Airport after airline officials said her coronavirus test result was out of date.
From Monday, travellers arriving in the UK, whether by boat, train or plane, have to show proof of a negative Covid-19 test to be allowed entry.
The test must be taken in the three days before travelling.
For those with connecting flights, the test must be 72 hours before your final departure point to England.
Anyone arriving without one faces a fine of up to £500.
Mrs Stone originally travelled to Cuba in February 2020 to see family. The British Cuban dual national was unable to fly home to the UK when Cuba closed its borders in March.
The family say she had several previous flights cancelled before finally being able to leave this weekend. She hasn't been able to see her four children or her husband Trevor in 11 months.
The government are understood to be speaking to Air Europa to try to get Mrs Stone home. Carriers have been told that they should permit stranded passengers to board and will not be fined for doing so.
While Mrs Stone has been caught out by the new restrictions for incoming travellers, the first day of the new regulations appeared to go smoothly.
'Crying her eyes out'
Mrs Stone left Jose Marti International Airport in Havana, Cuba, on Sunday night to fly back to the UK via Madrid.
She took a Covid test on Thursday to be guaranteed a result by Saturday. It was negative and Mrs Stone was able to board the plane from Cuba.
However, on arrival at Madrid-Barajas Airport, Mrs Stone says she was stopped from boarding the next leg of her journey to London Gatwick by Air Europa staff, because her test had been taken more than 72 hours before the final flight.
"She's crying her eyes out," says Trevor Stone, her husband. "I feel absolutely helpless. She doesn't have any Euros as she wasn't meant to stay in Spain. The authorities have given her no help whatsoever, we are just trying to understand what to do.
"She took her test 72 hours before the start of her journey, but had to take a connecting flight onwards. There would be no other way to do it, it is not physically possible."
In the meantime, Mr Stone says he has been home-schooling their four children on his own through the pandemic.
"We are just desperate to get her home - I'm so worried about her and after 11 months, she really wants to see her children," he added. "We haven't done anything wrong, I don't know what to do or who to turn to."
A Department for Transport spokesman said: "Passengers travelling to the UK must provide proof of a negative coronavirus test which meets the performance standards set out by the government in the guidance published on gov.uk.
"The type of test could include a PCR test or antigen test, including a lateral flow test. Anyone who cannot provide the necessary documentation may not be allowed to board their flight."
Air Europa and Madrid Airport have been approached by the BBC for comment.