Inaccurate design calculations, inferior steel and bad workmanship meant the disaster in 1994 at the port of Ramsgate, in Kent, was an accident waiting to happen, it was claimed.
Six people were killed and seven others seriously injured when the walkway fell more than 30 feet to the ground in the early hours of 14 September.
The Health and Safety Executive is prosecuting four companies under the Health and Safety at Work Act in connection with the accident.
Lloyds Register of Shipping yesterday pleaded guilty to a charge under the Act, which alleged that they failed to ensure the safety of passengers in checking the equipment.
Two Swedish companies, Fartygsentreprenader AB and Fartygskonstruktioner AB, have entered not guilty pleas to the same charge, but are not represented in court.
Port Ramsgate Ltd has pleaded not guilty to one charge under the Health and Safety at Work Act and one under the Dock Regulations Act.
Hugh Carlisle QC, for the prosecution, told the jury of seven men and five women at the Old Bailey that the walkway collapsed just four months after it had been installed at the port, which operates a passenger ferry route to Belgium.
Just one steel pin held the walkway in place, connecting the huge ferries with the terminal building. At the sea end of the walkway four sliding feet were supposed to keep it in place and allow for movement of wind, tides and waves.
But, claimed Mr Carlisle, inferior steel, inaccurate design and substandard welding aligning the feet to the walkway caused the accident. He told the jury the walkway was too rigid and the feet too small to support the loads.
"The design was inherently unsafe and it has to be said that this was a design that was both inept and incompetent. It was an accident waiting to happen and it happened earlier rather than later because of poor welding," he said.
Steven Jones, 34, from Manchester, and 42-year-old Jason Dudley, from Epping, Essexwere killed along with four other people from France, Italy and Belgium. Among those injured were British, Japanese, American and French tourists.
Mr Carlisle told the jury that the burden of proof would rest on the three defendants to prove they had done as much as they reasonably could to ensure the safety of people using the court and the walkway.
All four companies face an unlimited fine or the possible imprisonment of individuals if found guilty.
The trial is expected to last three to four weeks.