UK Border Force officers will face violent attacks after the government told them to take the fingerprints of migrants who try to board Eurotunnel trains, a trade union has said.
The Home Office order was made to staff at the terminal in Coquelles, near Calais, after a spike in small boat crossings to the UK by migrants.
Trade union ISU said both staff and migrants will get hurt.
But the Home Office said effective risk assessments were made to minimise harm.
BBC Radio 4's File on 4 programme spoke to some of the hundreds of migrants living rough in Calais during the coronavirus lockdown, who said unbearable conditions were driving them to attempt to cross the Channel in small boats.
Since the lockdown began in March, more than 1,000 migrants have reportedly crossed to the UK on small boats.
The total for this year is already estimated to have exceeded the number who reached the UK in the whole of 2019, despite a pledge made last year by Home Secretary Priti Patel to ensure small boat crossings become an "infrequent phenomenon by spring 2020".
Now the home secretary has ordered UK border control officers, working at the Eurotunnel terminal in Coquelles, in the "pas de Calais" area, to take fingerprints of migrants they find trying to board trains illegally.
The Home Office said registering fingerprints may assist in returning people to France - via an EU law called the Dublin Regulation - should they later make a successful crossing into the UK.
But Lucy Moreton, of the ISU, a union representing front-line officers, said the move would spark violence as migrants try to avoid being registered in France.
"That is quite a challenge, particularly if they really don't want to have their fingerprints taken," she said.
"We don't have digital fingerprint recorders, we only have wet ink. So you have literally got to hold their hands and roll their fingers from side to side to get a print.
"That's quite a lot of avenue to fight back if that's what they want to do. I'm very concerned about the levels of violence that will result and the fact that there will be, eventually, staff and migrants injured."
Migrants used to have their fingerprints taken by UK Border Force officers at the port in Calais, but the practice was stopped in 2010 and was also later abandoned at Coquelles.
Ms Moreton said: "All it resulted in was a lot of physical violence and individuals - both our staff and migrants being injured. And migrants deliberately harming themselves in order to damage their fingertips so that their fingerprints can't be taken.
"That's not something that anyone wants to be forced to do."
The Home Office told the BBC people fleeing persecution should stay in the first safe country they entered and there was "no reason why they need to make an often dangerous trip" to claim asylum.
It added the Border Force has robust risk assessments in place to minimise the risk of harm to both migrants and officers.
File on 4's The Perfect Storm is on BBC Radio 4 on Tuesday 9 June at 20:00 BST and available afterwards on BBC Sounds.