At least £830 million has been lavished on a failed project to improve security at Britain's borders - leaving authorities with a creaking system for screening terrorists and criminals that collapses twice a week, a report has revealed.
It also emerged that millions of people are travelling to the UK without any information being collected about them in advance of their arrival.
The flaws were exposed as spending watchdogs delivered a scathing assessment of the controversial e-Borders project.
The scheme was devised in 2003 to enhance checks on those entering the country by air, rail and sea by gathering and processing data on passengers before they reach the border.
Since then the Home Office has racked up costs totalling hundreds of millions of pounds, including £340m on e-Borders, a £150m settlement following a legal dispute after the original contract was cancelled in 2010 and another £303 million on subsequent programmes.
Despite the outlay the overhaul is not expected to be complete until 2019 - eight years behind schedule - while the total cost to taxpayers is set to exceed £1 billion.
Because of the delays, a database used to alert border staff to known terrorists and criminals - which was developed 20 years ago and was only expected to last for seven years - will not be retired from use until March 2018.
The National Audit Office (NAO) report said although upgrades to the Warning Index system have improved its resilience, it is "still far from good" and suffers an average of two "high priority incidents a week".
These include episodes where a component of the system is not available or where it is inaccessible at 30% or more control points at a port or airport.
Keith Vaz, chairman of the Home Affairs Select Committee, described the report as a "devastating indictment" of e-Borders, which he labelled a "£1 billion waste of money".
He added: "The real concern is the warnings index, which with two priority incidents a week is still clearly unfit for purpose.
"With the terrorism threat level currently at severe, a failure to properly cover millions of people entering the country without having passenger information in advance gives a green light to people who wish to come to the UK for illegal or dangerous activity."
There are contingency systems in place for technical failures and the incidents do not relate to the "core infrastructure" of the index, the Home Office said, adding that no passengers pass through the border without being checked against the database and other watchlists.
The Warning Index operates alongside another facility called semaphore, which collects wider travel information.
A failure to develop an integrated system has contributed to "highly manual and inefficient" border operations, while £89 million has been spent on "improving vital systems that e-Borders should have replaced", the report said.
Staff examining passports were said to be left to check details against lists manually, while officers alerted to potentially suspicious vehicle registrations have to check licence plates against printed A4 sheets.
The e-Borders proposals aimed to collect passport information covering 95% of inbound passengers before they reach arrivals desks by 2010, rising to 100% by March last year.
However, the rate stood at 86% in September, indicating that details relating to one in seven passengers - the equivalent of 16.5 million people per year - are not available for analysis until they reach the country.
Data coverage for outbound journeys has reached 100%, but the NAO said: "The Department is still not receiving inbound passport data from the majority of ferry and rail passengers."
Delivery plans for e-Borders were "too ambitious", while there were "unrealistic assumptions" about an initiative requiring more than 600 air, ferry and rail carriers to supply data, the study concluded.
Amyas Morse, head of the NAO, said some "valuable capabilities" have been added to border defences, but added: "Since we are now in 2015, with the Home Office still not having delivered the original vision after expenditure of £830 million, I cannot view e-Borders as having delivered value for money."
Immigration minister James Brokenshire said protecting the borders is "our top priority" and systems are "working effectively to keep our citizens safe and our country secure", with the Warnings Index operating at a "higher capacity than ever before".
He added: "The e-Borders programme was set up under the Labour government and when that contract ended in 2010, our immediate priority was to invest in stabilising the crucial but old-fashioned systems, to tackle the fast-evolving terrorist, criminal and illegal immigration threats faced by the UK.
"Every passenger arriving in the UK is checked against a range of watch lists."