The government is being urged to be more compassionate to Syrian refugees who face deportation because they have travelled to the UK from other European countries to join relatives.
Under EU rules, refugees are meant to claim asylum in the first European state they enter.
Charity Migrant Voice wants Syrians who have arrived illegally, but have close family here, to be allowed to stay.
The Home Office said each case was considered on its merits.
Migrant Voice says it is seeing an increasing number of Syrians - and other nationals - who have come to the UK to join family, but face deportation because their new life began in another EU member state.
The government is implementing what is known as the Dublin Regulation.
Nazek Ramadan, the director of Migrant Voice, said it was concerned at the way the government was implementing the regulation.
"It can use its discretion on compassionate grounds to offer sanctuary and asylum to people who have family connections.
"But the government is being rigid and it's leading to many families being divided."
One of those the charity is supporting is Marwan, a 33-year-old man who arrived in the UK illegally two years ago and has been fighting to stay ever since.
He said he had been imprisoned and tortured by the ruling Assad regime in Syria.
He managed to escape from the country and then paid thousand of pounds to people smugglers for a treacherous boat journey to Europe.
He and his fellow travellers who survived eventually came ashore in Italy. He said he didn't realise that under the rules Italy would be his designated European home state.
"I don't have any family there," he said. "And in our culture you gather your family around you because they offer you support."
Marwan wanted to join his brother, sister and other relatives in the UK.
He headed for France and, after a number of attempts, arrived in the UK illegally in a lorry, but hoped that, because of his family connections, the authorities would look favourably on his application to stay.
But he said he has been told he must return to Italy.
He wants to eventually start a business in the UK, as he did in Syria, to provide for his wife and four children, who are still there. He is desperate to get them out.
"I came here not to save myself but to save my family," he said. "The whole point of me coming here is so that I can find a safe place for my children.
"I've already spent nearly three years in this country. I don't have enough time to go back and start the whole process all over again.
"What if I lost my family? If my children die in a war, what is the point of me living?"
The Refugee Council is also calling on the government to respond positively to cases like Marwan's.
Chief executive Maurice Wren said: "The government should be doing everything in its power to reunite families divided by war, rather than deliberately and permanently separating them."
The Home Office said it "will not shoulder the burden of asylum claims" which should be processed by other countries.
"We firmly believe in the established principle of the Dublin regulation, that those in need of protection should seek asylum in the first safe country they enter," a spokesman said.
In September, the prime minister pledged that over the next five years Britain would accept up to 20,000 Syrian refugees.
It will take them from camps in countries bordering Syria, not those who are part of the exodus to Europe.