A campaigner involved in a protest on a cargo ship claimed he believed it was carrying illegal genetically modified crops, Cardiff Crown Court has heard.
The ship was carrying animal feed to Bristol
Greenpeace had claimed the MV Etoile was carrying genetically modified animal feed.
The 123,000-tonne ship, one of the largest sea-going bulk cargo carriers, was eventually able to dock after the two-day protest in June 2004.
Ten men and three women deny a public nuisance charge. The case continues.
Ben Ayliffe, 28, a Greenpeace employee involved in researching GM organisms, told the jury that the aim of the protest was to stop the shipment coming into Avonmouth Docks in Bristol.
The group were aiming to get it to return to the USA.
He said he believed the protest's aims were reasonable, "given what we thought was on board the Etoile, absolutely".
"I absolutely, 100% believed there were illegal, unapproved GM crops on the Etoile when we stopped it."
He claimed that in the US genetically modified and non-GM crops are "thrown together" and the UK's Environmental Protection Act was clear that GM material had to be approved.
He added that "anything that is not approved is not allowed in".
Mr Ayliffe said he did not believe the law was being enforced properly.
"My experience with the GM industry is that they tend to close ranks," he said.
"They don't like studies leaking out to the public about crops that aren't meant to be there."
During the incident the RNLI and the Maritime and Coastguard Agency were bought in and South Wales police had deployed more than 80 staff to deal with the incident.
A police helicopter was deployed on two occasions.
The defendants include: Andrew Taylor, 35, of Stroud, Gloucestershire, Janet Miller, 49, and Huw Williams, 38, both of Buxton, Derbyshire, Tim Hewke, 45, of Sittingbourne, Kent, Nicola Cook, 37, of St Cross, Suffolk, and Cedric Counord, 28, of France,
They appeared with the following defendants, all from London: Allen Vincent, 42, of Peckham, Michele Rosato, 33, of Bow, Rachel Murray, 31, of Highbury, Jens Loewe, 36, Richard Watson, 40, and Ben Ayliffe, 28, all of Islington, and Frank Hewetson, 40, of Kensal Rise.