Fewer people in Lincolnshire became British citizens last year after participating in special citizenship ceremonies, figures show.
The think tank British Future said many ceremonies have been delayed across the UK due to the coronavirus pandemic, leaving applicants waiting for the security that citizenship provides.
Home Office figures show 254 people attended citizenship ceremonies in Lincolnshire in 2020 – down 22% from 327 the year before.
It means that since the figures were first published in 2004, 5,115 people have gained citizenship in the area.
Just under 75,000 people took part in citizenship ceremonies nationally last year, including around 1,000 at British consulates abroad.
This was a drop of 34% from 2019, and the lowest annual figure since 2004.
The events are the final step in the process to full citizenship and being able to obtain a British passport, but were suspended for large parts of 2020 due to Covid-19.
Participants are asked to make an oath of allegiance to the Queen and pledge to respect the rights, freedoms and laws of the UK.
They are then presented with a certificate of British citizenship and a welcome pack.
Madeleine Sumption, director of the Migration Observatory at the University of Oxford said: “During the pandemic, the process of becoming a British citizen has been slower and more uncertain than in the past.
"This is partly because of the difficulty scheduling a citizenship ceremony, which remained a requirement even though many local authorities were not offering ceremonies due to Covid-19."
In the year to June, citizenship applications rose by 36% nationwide, but grants of citizenship increased by just 5% over the same period.
Steve Ballinger, director of communications at the think tank British Future, said: “Registrars worked hard but Covid meant many citizenship ceremonies were delayed – and adults don’t get their papers until they have attended the ceremony.
"People were left waiting for the security and sense of belonging that citizenship brings."
He said the pandemic must not be an excuse to sideline citizenship ceremonies, with a 2020 report for the independent citizenship inquiry calling for them to be revitalised.
The Institute for Public Policy Research think tank said the costs of citizenship have increased considerably in recent years, and wants the current process reformed.
Marley Morris, IPPR associate director for migration, trade and communities, said: "For many, citizenship ceremonies are an important symbolic moment, yet they have a low public profile.
"Councils should therefore explore ways to celebrate citizenship ceremonies more widely in their local communities."
A Home Office spokesman said local authorities paused in-person ceremonies to put the health of the public first, but they have now restarted.
He added: “We continue to work closely with local authorities to ensure anyone who requires a British citizenship ceremony can attend one as quickly as possible."