Passport Office workers have launched a five-week strike in the increasingly bitter civil service dispute over jobs, pay, pensions and conditions.
More than 1,000 members of the Public and Commercial Services union (PCS) at eight sites are walking out in an escalation of the long-running row.
A picket line was staged outside the office in Glasgow on Monday morning after the union said those taking action will be supported by a strike fund.
Those in Renfrewshire are being warned that there may be delays with applications.
PCS general secretary Mark Serwotka has written to the Government calling for urgent talks in a bid to resolve the dispute.
He has accused ministers of treating their own employees differently from others in the public sector after negotiations were held with unions representing health workers and teachers.
The union is stepping up strikes, with a nationwide walkout of more than 130,000 civil servants planned for April 28.
The Home Office said the Passport Office has already processed more than 2.7 million applications this year, adding over 99.7% of standard applications are being processed within 10 weeks, with the majority of those delivered to customers well under this timescale.
There are currently no plans to change official guidance which states that it takes up to 10 weeks to get a passport.
Meanwhile, Heathrow said the airport continues to operate as normal and security lanes are free-flowing despite an ongoing strike by security guards.
Contingency plans are “working well” an airport spokesman said, adding: “There have been no last-minute cancellations at Heathrow due to these strikes.
“Any cancellations were agreed and actioned at the start of the week, giving passengers advance notice.
“Any additional cancellations today are an airline decision and not connected to these strikes.
“This could include a number of factors such as aircraft issues, crew shortages, weather at outstations or air traffic control issues like the ongoing strikes in France.”
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here