Just weeks ahead of the peak holiday season, thousands of people across the UK are estimated to be facing severe delays having their passports processed, with the agency currently dealing with about 465,000 renewals and first-time requests across the UK.
The main Passport Office union said the delays were down to job cuts since 2008, claiming it had set the agency's chief executive a June 30 deadline to resolve staffing shortages before progressing with its industrial action threat.
Amongst those affected by the delays are Glasgow GP Paul Costello, his wife Louise and their seven-month-old son Jude, who have been waiting almost three months for a passport for the baby.
The family have been based in Brisbane, Australia, for the past year, with Louise giving birth in November. But despite official UK Government guidance claiming a six-week turnaround, the family have been waiting nearly three months with no indication of when the document will arrive.
Mr Costello is due back in Glasgow in 10 days, with no cover available for his GP practice in Knightswood, but the UK High Commission in Canberra no longer issues passports - not even an emergency document for a child's first passport.
Mrs Costello said: "The only information we've received is to tell us the documents have been received. Nothing more.
"I've now accepted Jude and I will have to remain here in Brisbane while Paul returns home, meaning extending leases on the property. But I don't even know how long that will be.
"I'm a new mum and have been very focused, planned my timing, went with the guidance. It never occurred to me I'd be effectively stranded on the other side of the world."
Mr Costello said: "The fact of the matter is we've been left high and dry. Louise faces being left here alone, which is anything but ideal, and we've no idea how long we'll be apart. There's a serious financial cost too."
A Passport Office spokesman said embassies and high commissions phased out issuing passports in March, with guidance given on a country-by-country basis.
"He added that timings had also changed due to security checks, and these were taking significantly longer than UK applications.
Passport Office Paul Pugh said the service was "experiencing exceptional early summer demand for passports, in part due to the improving economy and a rise in holiday bookings" and that "over 99 per cent of straightforward applications have been processed within four weeks".
But Lynn Henderson, Scottish secretary of the PCS union, said: "We didn't have such delays during boom times when everyone was going on holiday. There's an easy explanation why, and that's the cuts to the service."
At Westminster there were angry exchanges between the Coalition and Labour over the delays.
During a fiery Prime Minister's Questions, Ed Miliband called on David Cameron to "get a grip", telling MPs: "Tens of thousands of people are finding their holidays are being cancelled because they are not actually getting a passport."
But Mr Cameron hit back, accusing Mr Miliband of trying to frighten people, claiming fewer than 10% of the 300,000 extra passport applications were not being processed within the normal three-week target and that 250 staff had already been redeployed to the front line to get through the backlog.
Glasgow ceased processing the documents six years ago.