The ability of port officers to defend Britain against terrorists is being hindered by shortcomings in information they receive about passengers, a watchdog has warned.
David Anderson QC, the independent reviewer of terrorism legislation, said there is "widespread dissatisfaction" with the quality of manifests on the part of police at both Channel and Irish Sea ports.
Meanwhile, it was also revealed that the activities of preacher Anjem Choudary were referred to the Crown Prosecution Service for review on 10 occasions over 13 years - without a prosecution being brought.
Mr Anderson highlighted his findings from visits to sea ports in his annual report, which was published on Thursday.
He wrote: "The common and strongly-expressed refrain from ports officers on the ground was that they could do their jobs more effectively if they had better advance information about passengers arriving (and departing) by sea."
At the port of Dover he was told that inbound ferry manifests may be provided only shortly before ferries dock and their quality varies between carriers.
Mr Anderson said the absence of detailed passenger information, supplied hours in advance, contrasts with the "generally good" advance information available at airports.
Legislation is in place for the collection of passenger data, the study said.
It added: "It is however evident from my conversations with officers at affected ports that their ability to defend the country from terrorists (and to identify possible outbound terrorists) is impaired by a lack of the timely and comprehensive passenger manifests that would assist them in determining which passengers should be stopped and questioned."
Mr Anderson said steps should be taken to ensure that the quality of manifest data is improved.
His conclusion follows a warning by a review of counter-terrorism arrangements in October that Britain's borders are not as secure as they should be.
The official threat level for international terrorism in the UK currently stands at severe - meaning an attack is "highly likely".
A Home Office spokesman said: "Border Force is already world-leading in its analysis of passenger data to stop known or suspected terrorists from travelling to the UK.
"All ferry passengers seeking to enter the UK are subject to a range of checks, including against a range of security and immigration watch lists, either at the juxtaposed controls in France or upon arrival in the UK."
Elsewhere the report referred to the case of Choudary, who was jailed for five and a half years in September for drumming up support for Islamic State (IS).
Mr Anderson wrote: "It is entirely fair to ask why the law did not catch up with Anjem Choudary sooner.
"I raised the issue with the CPS and have been made aware in response of no fewer than 10 occasions between July 2002 and August 2015 on which his activities were referred to the CPS for review, but without prosecution ensuing."
He said Choudary's case is unusual, adding: "But even assuming that the issue is a general one, it would be wrong to assume that there is an obvious legislative solution."
A CPS spokesman said: "In all cases we apply the full code test as set out in the Code for Crown Prosecutors.
"If there is not enough evidence to provide a realistic prospect of a conviction then we cannot prosecute.
"Once we were satisfied that a case passed to us by the police had met the evidential test, and was in the public interest to pursue, we undertook a successful prosecution that led to Choudary's conviction in August."
Choudary, 49, has applied for permission to appeal against his conviction.