Twelve Church of England bishops have accused the government of criminalising “Good Samaritans” who seek to save the lives of migrants crossing the Channel in small boats.

The bishops, who speak on asylum and refugee issues within the church, said the nationality and borders bill before parliament is an “affront to justice” by putting the saving of life under a legal penalty.

Record numbers of people have crossed the Channel in small boats this summer, including more than 1,500 people last week.

Under the bill, migrants could face up to four years in prison. Plans put forward by Priti Patel, the home secretary, will require Border Force vessels to turn back migrant boats rather than bringing passengers safely into the UK.

International and maritime law says assistance must be given to people in distress at sea. Article 98 of the UN convention on the law of the sea 1982 mandates that every state requires its ships “to render assistance to any person found at sea in danger of being lost”.

Gérald Darmanin, the French interior minister, has said France “will not accept any practice contrary to the law of the sea”.

In a letter published by the Guardian, the bishops said: “We are deeply concerned about the government’s approach to migrant crossings of the Channel.”

The bill “would criminalise not only attempts to cross the border irregularly, nor even simply people smuggling, but even those who take part in the rescue of boats in distress at sea.

“This would require those who see asylum seekers at risk to choose between ignoring a moral imperative (also established in maritime law) to assist them, or to risk prosecution and imprisonment.

“This amounts to a criminalisation of the Good Samaritan who did not pass by on the other side, and an affront to justice to put the saving of lives under any sort of legal penalty.

“The new ‘turn back’ policy, which will see boats forcibly returned to France, also raises significant moral concerns. It starkly increases the risks at sea and endangers the lives of those attempting the crossing.”

Increased security in the Channel “is demonstrably failing to deter attempted channel crossings”. Rather, Channel crossings “are actively being made more dangerous, at greater cost in human life, by the government’s own policies.”

A multilateral approach, promoting safe routes and valuing human life and the “dignity of the vulnerable”, was needed, the bishops said.

Paul Butler, the bishop of Durham, said: “We agree with the home secretary that we need a better and more efficient asylum process, and we agree on wanting to stop human trafficking.

“But the answer is more designated safe routes. The situation in Afghanistan has demonstrated that it’s possible to identify the most vulnerable people, sort out the necessary paperwork and set up safe routes.

“In Afghanistan, we have seen the story, seen the horror. With a lot of the folk in Calais, we don’t know their stories. If we did, levels of sympathy and compassion would increase.”