Even if overseas holidays are permitted from 17 May, prospective holidaymakers may find they are waiting four weeks beyond that date for a passport.
HM Passport Office says people should now allow up to 10 weeks when applying for a British passport. Previously the guideline was three weeks.
An application made on 7 April could take up to 16 June 2021.
In 2020 the office issued only four million passports, compared with seven million in a normal year, as international travel collapsed.
At present it is illegal to go abroad except for essential purposes. But with international travel possibly allowed from 17 May, it is feared that a surge in demand could lead to waiting times extending well into the summer.
The backlog is being exacerbated by complicated new rules on passport validity brought about by Brexit.
Previously British passports were valid up to and including their expiry date. But since 1 January 2021, the government says they must have at least six months to run.
Passports issued for over 10 years are subject to extra restrictions in the European Union.
The director-general of the Passport Office, Abi Tierney, said: “It is vital those who may need to apply for a new passport do so now…