More than 1,200 migrants have arrived in Dover in just four days as smugglers exploit the heatwave engulfing Europe.

Some 310 people arrived on Sunday, taking the total this year past 11,000.

And Border Force will be concerned about the growing number of people arriving as the weather improves.

Home Office records show more than 300 asylum seekers have arrived on three of the last four days.

It comes as Labour was forced to rewrite Keir Starmer‘s flagship migrant deal with France amid fears migrants were just returning in lorries.

The “one-in-one-out” agreement said asylum seekers caught in a boat could be returned.

But smugglers began trafficking migrants already deported from the UK back in lorries.

So the Home Office agreed with Paris to create “returnee” cases.

Officials believe at least four migrants deported from France travelled back in a lorry over a two-week period in March.

The UK has removed to France 921 migrants who arrived on small boats since the treaty came into effect on 6 August last year, which represents 3.5% of all such arrivals in the same period.

Home Secretary Shabana Mahmood, in a letter to Laurent Nuñez, the French interior minister, said: “Following our recent meeting which allowed us to observe the quality of the cooperation established under the agreement … I wish to propose an addition to the objectives of the agreement, explicitly adding the objective of deterring clandestine returns to the UK by individuals previously transferred to France under the agreement.”

Organised crime gangs have spread out even further along the coast, as far south as Dieppe, to avoid French law enforcement efforts.

Migrants are now being forced to spend 20 hours on dangerously overcrowded dinghies which often fill up with water and petrol.

Dr Peter Walsh, senior researcher at the Migration Observatory at the University of Oxford, said: “Arguably this is the strongest indicator that French and British enforcement efforts on the French coastline are having an impact because they are changing smuggler behaviour.

“That does suggest departing from the usual places in Northern France is becoming less viable, but it also means the boats and migrants are taking potentially longer and more dangerous journeys.”

“Organised crime are adapting their tactics to stretch greater attempts by law enforcement particularly in France, in particular by launching from the Belgian coast,” said John Vine, the former Independent Chief Inspector of Borders and Immigration.

“Although the figures for crossings are down on last year , the fact remains that this trade will continue unless and until there is a credible deterrent to tackle the demand.

“Enforcement alone will not end illegal small boat crossings.”