As parts of England could see temperatures top 30 degrees Celsius over the next few days, Ben Bano co-director of Seeking Sanctuary has urged the government to treat the migrants with dignity and respect.
He said: "The answer to this problem is not just about increased surveillance techniques and stopping people from starting their journeys - it is also about addressing the appalling conditions and harassment which they experience in and around Calais that lead them to feel that their only option is to risk their lives making these desperate attempts.
Boris Johnson's warning that migrants would be sent back if they crossed the English Channel to the UK have been branded as "inflammatory" and "misleading" by campaigners.
The Prime Minister's comments came after a surge in attempts - with nearly 100 people taking to small boats to try to make the crossing from France to the UK in one day.
Bano added: "There is nothing illegal about seeking sanctuary from violence and oppression.
"If these exiles were treated with a degree of humanity and respect, rather than being endlessly harassed in France they are less likely to resort to traffickers and risk death by using unorthodox means to try to reach the UK."
Home Secretary Priti Patel is to seek talks with her French counterpart, the Interior Minister Christophe Castaner, to address the crisis.
Mr Johnson told reporters: "Clearly the most important thing is to stop them coming across from France, so we are working very closely with the French authorities.
"The point I would just make to people thinking of making this journey - one, it is very hazardous, you may think the weather looks great but it's a very, very dangerous thing to do.
"The second thing is - we will send you back. The UK should not be regarded as a place where you could automatically come and break the law by seeking to arrive illegally.
"If you come illegally, you are an illegal migrant and I'm afraid the law will treat you as such."
The UK has a legal obligation under what is known as the Dublin Regulation to ensure asylum applications lodged are examined and considered.
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