French riot police deployed in northern France under a £660m deal with the UK are authorised to use water cannon against asylum seekers, the Guardian has been told.

Two specialist policing units, including a 50-officer riot squad, have begun working to prevent asylum seekers and people smugglers from launching small boats under the landmark UK-France deal in time for the summer months.

Informed sources said that French riot officers from the Compagnies Républicaines de Sécurité (CRS) are free to request the use of water cannon, CS gas and batons as part of their public order powers.

Use of water cannon, which can fire a high velocity stream powerful enough to knock people off their feet, is prohibited in Great Britain owing to concerns that they are a threat to “policing by consent”. However, they can be deployed in Northern Ireland and were used last week to quell anti-immigration protests.

Any use of water cannon would be “sickening”, a refugee charity said. Steve Smith, the chief executive of Care4Calais, said: “This would be a sickening escalation in the state violence being waged against refugees here in Calais.

“The UK government treats refugees as second class citizens, and funding these types of indiscriminate, violent tactics against refugees in France would underline that. If the use of water cannon is prohibited in Great Britain, then we shouldn’t have a government backing its use against communities in another country. It’s inhumane and the UK government should be ashamed.”

The announcement that French riot police and 75 officers from the Compagnie de Marche had been deployed on French beaches was made by the prime minister, Keir Starmer, at the G7 summit in Évian-les-Bains on Wednesday.

“These elite units are just one element of the landmark deal between our countries that is taking our collaboration to the next level,” Starmer said.

A permanent specialist CRS police unit “specially trained in the use of riot and crowd control tactics” has been deployed to maintain and restore public order, a government statement said.

“This team are better equipped to respond to hostile crowds and evolving smuggler tactics and stop illegal migrants in their tracks,” a Home Office statement said.

French riot police have deployed water cannon on many occasions, including earlier this month to quell disorder after Paris Saint-Germain’s victory in the Champions League final.

They were photographed in Calais and Dunkirk last July using CS gas and batons. They were attempting to stop gang members and asylum seekers from reaching small boats in shallow waters.

The CRS has faced allegations of brutality and racism. In February, nine officers from the force were found guilty of beating peaceful protesters who were sheltering from teargas during the “gilets jaunes” anti-government demonstrations in 2018. Two years ago, leaked WhatsApp messages showed CRS officers had exchanged an image of a sinking small boat laden with migrants.

Water cannon deployment on the British mainland was ruled out by Theresa May, the then home secretary, in 2015. She told MPs they “have the capacity to cause harm”.

Police in Northern Ireland still have access to six water cannon. They were used last week to disperse a crowd of about 300 people who burned a truck and threw bricks and petrol bombs close to the Sandyknowes roundabout near Newtownabbey, 8 miles (13km) north of Belfast.

The home secretary, Shabana Mahmood, first announced plans to employ French riot police permanently on Channel beaches last month as part of a set of measures designed to tighten security in northern France. It covers the construction of a new detention centre, the expansion of maritime tactics to intercept boats in shallow waters and a 40% increase in officers deployed on French beaches.

Downing Street said a surge in joint law enforcement operations since the deal was agreed had already seen 40% of attempted crossings halted in May. Despite these efforts, 2,726 people still made the crossing to Britain in May, about 1,000 fewer than made the journey over the same month last year.

When approached by the Guardian, Home Office sources said the money that will pay for the French riot squad will come from French government funds, although their deployment was agreed under the £660m deal. “Policing tactics are a matter for the French authorities,” a source said.

The Home Office has been approached for a comment. The CRS has not responded to several requests for a statement.