The notice advises asylum seekers how to fix broken boat engines and send GPS signals and contains links to weather forecasts.
The poster is thought to have been put up by activists from the Watch The Channel group.
It has been condemned for “encouraging” migrants to make the dangerous crossing.
Dover MP Natalie Elphicke said: “This is dangerous and irresponsible communication being given out in the camps by social media and internet-based enablers.
“The Channel crossings are incredibly dangerous and this puts lives at risk.The best advice for people is to stay safe on land in France.”
Alp Mehmet, chair of Migration Watch, said: “Not only is travelling across the Channel by small boat illegal, this poster omits all the dangers and overlooks the lifethreatening risks that they are effec- tively encouraging people to take. They should be ashamed of themselves for doing this. It is bizarre that there are people claiming to help people by actually helping them to risk their lives like this.
“I am horrified there are people that are so misguided, wrapped up in their own self-righteousness, that they are going to this extent to encourage people to climb on to these small boats.”
The document is written in English, Kurdish, Farsi, French, Kurmanci, Pashto and Sorani. It was stuck on a food truck in a camp near Dunkirk, northern France.
It advises asylum seekers to “bring emergency foil blankets, food and water for one day” and provides them with France’s emergency 112 number.
It also gives migrants a guide to restarting an engine if it breaks down, adding: “Be safe, always wear a life-vest, always stay seated and calm and watch out.” And if someone falls in the water, migrants are advised to “immediately point and keep pointing to the person so you don’t lose them in the waves”.
Watch The Channel describes itself as “a group of activists dedicated to supporting people who are forced to cross the Channel by irregular means”.
Children whose home is hellish ‘Jungle’ camp
A woman and children gather by a tent in a squalid migrants’ camp in Northern France yesterday – as a desperate Iraqi asylum seeker described his first week of hell there.
Barber Rawan Sadiq Ahmed, 36, had lived and worked in Belgium for 15 years when his battle for asylum suddenly ended.
He now lives in a tent in the Grande-Synthe camp, which is nicknamed the “Jungle” and is regularly targeted by French police.
Rawan said: “I couldn’t get papers for 15 years. Then they just kicked me out. I lived there. I worked there. Now, I have no home, no work, nothing.
“I am very scared. I don’t like living here but I don’t know what to do. It is dangerous here. There are children crying and you don’t eat or sleep.”
Youngsters in the camp live in tents and huddle by bonfires for warmth.
Another migrant, Aryan Mustafa, 23, said: “We don’t have any food here, we don’t have a shower. We look like gypsies in this jungle.”
France is under pressure to give aid to migrants on the Northern coast so they don’t fall seriously ill or die.