A delivery company has apologised amid reports that people are facing lengthy delays receiving their passports and some have had to cancel travel plans in the run-up to Christmas.
TNT, which is owned by FedEx, has an exclusive contract to deliver travel documents for the UK Passport Office.
But there are reports of delivery dates being delayed multiple times, with some people still waiting after a month rather than the standard two days.
Passports have also been lost.
A FedEx spokesperson said: "We fully understand the importance of these shipments to our customers and are taking additional steps to provide timely delivery by adding additional resources to the operations in the UK.
"We apologise for any inconvenience caused."
Due to Covid backlogs, passports are already taking up to 10 weeks to process, the Passport Office (HMPO) has warned. But delays delivering the documents are adding weeks on top of this, according to complaints on Trustpilot and Twitter.
One person tweeted: "Paid nearly £300 for two urgent Fast Track passports with a guaranteed 7-day turnaround. 20 days later and nothing - Can't get through to TNT and @HM_Passport seems to have washed their hands of it. Really distressed."
Another said: "I have had three delivery dates and it hasn't turned up. Been on hold on the customer service phone line for nearly 3 hours yesterday. Was told it was coming today but when I checked it has now been moved to Monday."
An online petition has been started demanding an investigation into HMPO's £77m three-year contract with TNT, signed in 2019.
It states that thousands have experienced "significant delays" in getting their passports delivered in recent months, and many have cancelled travel plans as a result.
"Unlike many other couriers, TNT do not seem to provide real time tracking, nor can you provide delivery instructions," the petition says.
"People have experienced numerous failed delivery attempts, others have had their delivery date continuously pushed back with no end in sight. Some have even had their passport or application supporting documents lost by TNT."
It added that it was "nearly impossible" to get information from TNT and the Passport Office was seemingly uninterested in helping.
According to the Times newspaper, HMPO is turning to alternative postal companies. Quoting a source at the department, it said that in cases where TNT had lost a passport it was re-printing the document and sending it by Royal Mail instead.
FedEx blamed the delays on an increase in passport applications as Covid-19 restrictions continue to ease.
"This has resulted in a significant increase in the items to be delivered through our dedicated HMPO network," it said.
It added that it was working closely with HMPO to ensure recipients are informed of potential extended delivery times.
A HMPO spokeswoman said it expected "the highest standards from its partners" but that TNT was dealing with an unexpectedly high volume of applications.
"We are aware of some additional reported issues from customers related to missed deliveries by our delivery partners, and are working closely with our partners to establish the root of this issue," she added.