Chickens slowly freezing to death, being boiled alive, drowning or suffocating under piles of other birds are among hundreds of shocking welfare incidents recorded at US slaughterhouses, according to previously unpublished reports.

Among them are “inexcusable” violations, say campaigners, who ask if the USDA’s current system, where inspectors issue reports when they see violations, really works. One inspector, who asked to remain anonymous, questioned the impact of those reports.

A joint investigation between the Bureau and the Guardian looked at hundreds of inspection logs from the US Department of Agriculture (USDA) detailing incidents in poultry plants across the country. In recent years, inspectors recorded numerous incidents where:

Chickens suffocated to death beneath other chickens when they piled up on a conveyor belt that had stopped due to a mechanical failure.

Chickens drowned after entering the scalding tank while conscious.

Inspections revealed machinery failures and incidents of staff being inadequately trained.

Thousands of birds died of heat stress after travelling or being left waiting in trucks in temperatures above 32 degrees C, or alternatively, freezing to death in extremely low temperatures.

In one incident in January this year, more than 34,000 chickens froze to death while being held overnight outside a slaughterhouse in a truck.

The investigation's findings have fuelled fresh concerns that following Britain’s departure from the EU, a post-Brexit trade deal with the US could see the UK flooded with chicken produced to lower welfare standards. This follows last year’s transatlantic row over chlorinated chicken which prompted political interventions in both countries.

The records include hundreds of instances in which groups of chickens and turkeys are bludgeoned, suffocated, scalded, frozen or heated to death. Repeatedly the inspectors note that the plants have not put in place adequate protection from the weather while the animals wait. However, they also note that after many reports the plants carry out retraining for workers.

These violations were witnessed between 2014 and this year at some of the largest poultry processors in the country as part of the national inspection system. The records, kept by the USDA’s Food Safety and Inspection Service (FSIS), were originally obtained through a series of Freedom of Information Act requests by the nonprofit Animal Welfare Institute (AWI).