French police have waded into shallow waters off a beach south of Boulogne and used knives to slash an inflatable small boat – packed with men, women and children – that was wallowing, dangerously, in the waves.

All those onboard clambered to safety as the boat collapsed in chaotic scenes.

The intervention was highly unusual.

French police usually follow strict rules that bar them from going into the sea in case they put lives at risk.

“Let’s go in,” said one of the gendarmes, pulling off his body armour, and taking out a small knife. His colleagues took their heavy armour off, too, placing equipment in the back of a nearby police car before rushing into the water.

There had been some speculation that this rare incident could be evidence that the French police – under growing pressure to stop a surge of small boat migrant crossings to the UK – are changing their tactics.

But they have made it clear to the BBC that police have not adopted any new tactics in dealing with small boat launches, that the rules forbidding intervention in the water remains in place and officers must continue to prioritise safety on the beaches. They are allowed to intervene, however, if they believe lives are at immediate risk.

Well-placed sources in France have told us that the procedural changes now being considered will almost certainly focus on the use of patrol boats at sea to intercept the “taxi-boats” before they’re fully loaded, rather than on approving more aggressive interventions from police on the beaches.

The UK prime minister’s official spokesman said the images of French police destroying a boat were “a significant moment and we welcome this action”.

“We want to see tougher action taken, that’s precisely the focus of our work, it is the outcome of that close work that you’ve seen,” the spokesman said.

A few metres offshore, the boat itself was clearly in trouble. People were crowded around the outboard motor, which had briefly stalled but was being restarted.

Waves were breaking underneath the boat, causing it to lurch wildly, and there were loud screams from several children who were in danger of being crushed onboard.

Earlier, two large groups of people already wearing orange life jackets had emerged from the nearby dunes and rushed towards the sea.

In all there were probably 80 or 100 people. But when the first “taxi-boat” – used by the smuggling gangs to collect passengers from various points along the French coast – sped past perhaps 100m from the shore, it was clearly full already and did not stop to pick anyone else up.

A few minutes later, a second boat, with almost no passengers, came towards the shore, watched by a French coastguard boat further into the English Channel.

Initially, people were ushered forwards in organised groups, holding hands, and directed by one man who appeared to be leading events.

But as the inflatable boat turned and reversed towards the shore, there was a scrum as dozens of people scrambled to climb aboard in water that was at least waist deep.

At first the gendarmes declined to intervene and stood watching from the shore.

One officer repeated a now-familiar explanation to me – that they were barred from going into the water except to rescue people.