Passport processing delays could cost £1.1billion in cancelled trips this summer, experts warn.
It has been estimated there is only a 50% probability of a “successful and timely” passport renewal process for travellers, the Centre for Economics and Business Research found.
The think-tank used VisitBritain figures to come to its estimate.
There have been warnings of delays in processing applications as demand soars after the pandemic.
The Passport Office is advising travellers to apply at least 10 weeks in advance. It comes after the number of valid UK passports fell by 2.6 million over two years from 2019 as people delayed applications.
Shadow Home Secretary Yvette Cooper told the Commons last week two constituents’ honeymoon is at risk “even though they applied in plenty of time”.
Home Office minister Kevin Foster said the Government is “confident” it will not need to extend the 10-week target.
It comes after MPs last week blasted the "shambles" and said one woman had been waiting since January ahead of a holiday next week.
The rules are also more complicated because since Brexit, Brits can no longer travel to the EU on a passport that's about to expire.
Under Schengen area rules, your passport must be less than 10 years old on the day you enter, and valid for at least three months after the day you plan to leave.
The three-month rule does not apply to Ireland but does apply to some non-EU countries too - Switzerland, Norway, Iceland, Liechtenstein, Andorra, Monaco, San Marino or Vatican City.
Home Office minister Kevin Foster warned people hoping to travel abroad this summer should apply to renew outdated passports "as soon as possible".
He also told MPs the Government had no intention of letting the current 10-week processing time slide further.
But the Commons heard of one woman who has waited since January to receive her daughter's new passport, with services branded "either really very good or an absolute shambles".
SNP home affairs spokesman Stuart C McDonald asked: "All our constituents are having to cancel holidays, miss funerals, rearrange visits, with even a new 10-week target routinely being failed.
"What will be done to avoid this predictable mess getting worse?
"And can we be assured that the 10-week target will not be lengthened further as we approach the summer?"
Mr Foster replied: "First in terms of - I go back to the fact that we issued, we dealt with, a million passport applications last month alone.
"To put that into context, we usually deal with seven million in a whole year.
"I would point out that where people have compelling and compassionate circumstances such as a funeral, applications can be expedited and it has now been for some time we have been advising to allow for up to 10 weeks for an application to be processed, and last year sent 4.7million texts reminding people whose passports have expired to renew them."
He added: "But no, we don't have any intention to further extend that standard.
"We are at the moment processing most passports well within it, but we would advise people this is a very, virtually unprecedented surge in demand, and if people are planning to travel this summer we would advise them to get their application in as soon as possible."
Conservative MP Simon Hoare described the passport backlog as "unprecedented true, but foreseeable absolutely".
Passport Q&As
Q: Why are there passport delays?
A: One reason is the number of people applying for one. As international travel returns to pre-pandemic levels, the number of travellers submitting their forms has spiked in recent months.
Q: Can I can speed up my application?
A: There are two ways - the one week fast track service, or Online Premium Service. The Fast Track service costs £142 for an adult passport or £122 for a child passport.
The Online Premium Service costs £177 for an adult passport.
Q: Where else can I get information?
A: The Passport Office
has an adviceline on 0300 222 0000.
