Half of the number of trailers tested have been found to be defective, a Department for Transport report said.
The tests follow the death of Freddie Hussey, three, who was killed by a runaway trailer in Bristol in 2014.
His parents and Karin Smyth, MP for Bristol South, have been campaigning for better safety standards ever since.
As part of that the Driver and Vehicle Standards Agency (DVSA) carried out 800 spot checks on trailers between December last year and February.
The Labour MP said the government described it is a “milestone” while acknowledging “we have more more work to do”.
“This campaign is now at the heart of government,” Ms Smyth said.
“The government doesn’t do anything unless it has any data and we knew there was an issue here but we didn’t have any proof and now we have,” she added.
Freddie died on his way to nursery in Bedminster in January 2014 when a badly attached portable cabin came loose from a car, trapping him against a wall.
In their grief, his parents and Ms Smyth started campaigning for better safety standards.
In 2016, the DVSA published a video, starting a Tow Safe for Freddie campaign.
In April, a parliamentary group was set up to research what could be done to improve trailer safety.
As part of the work, the DVSA carried out spot checks on boat trailers, caravans and horseboxes.
These were done across 10 enforcement areas to gain a wide-ranging sample and found half of them had basic faults.
The report also stated that in 2017 there were 20 collisions involving trailers which resulted in injury or death because of a vehicle defect.