Livestock hauliers were found to routinely break EU laws for the protection of animals in transit.
It follows monitoring by welfare charities in Bulgaria, an EU entry point to Turkey and the Middle East.
The UK government said it would “take steps to control the export of live animals for slaughter” post-Brexit.
But some animal rights groups are sceptical, and call for a ban on journeys of more than eight hours. The BBC’s Victoria Derbyshire programme investigated their concerns.
UK exports of live cattle, sheep and pigs have tripled in value over the past five years, to more than £21m in 2016, according to HM Revenue and Customs. The market is fuelled by a desire among some religious communities for live animals and, in some cases, difficulties in rearing livestock and refrigerating meat.
Across the EU, total exports of live cattle and sheep were worth more than €4bn (£3.5bn) last year, according to Eurostat, the EU’s statistical office.
Exports of cattle and sheep from Europe to non-EU countries such as Turkey, with lower animal welfare standards, have increased in value by about 50% since 2012.
Christine Hafner, an animal investigator for welfare charities, has been working on the Turkish border.She said animals die every summer at the border because of long delays while transporters wait to clear Turkish customs.
The BBC saw widespread health issues among livestock being transported into Turkey, including respiratory problems, eye conditions, and animals suffering from exhaustion or dehydration.
A bull was found dead on the top deck of a transporter on the Turkish border, while its driver was waiting to collect customs papers.
During an incident witnessed by the BBC last month, a truck of 500 sheep became stuck between checkpoints for four days in high temperatures without access to food and water.
When it was unloaded in Turkey, at least one sheep was found dead inside.
Ms Hafner said: “When sheep are forced to stand for many hours, if not days, on a truck, in more than 35-degree heat, with their heads and backs touching the ceiling, it’s like an oven.”
The Bulgarian border authority told the BBC “the consignment complied with all requirements relating to animal welfare during transport”.