Postal services have been disrupted after workers took industrial action over the dismissal of a colleague.
The Luton sorting office handles about 150,00 letters a day
The staff went on an official 24-hour strike at the Royal Mail sorting office in Luton in the early hours.
The Communication Workers Union said a picket line was an "unavoidable action after negotiations failed".
A Royal Mail spokeswoman said 150 staff had been striking and volunteer managers were drafted in and "expected to maintain a lot of the service".
In a statement, the Royal Mail said: "We apologise to customers in the Luton area for any disruption to their service today.
"We are extremely disappointed some staff at Luton delivery office have decided to take industrial action for 24 hours and, with volunteers and other staff we are doing all we can to maintain deliveries and collections in the LU1 and LU2 postcode areas, which the delivery office covers."
Paul Troughton, of the Communication Workers Union, told the BBC: "It is the last alternative and there is nothing else we could do.
"I'm here, I'm willing to negotiate even at this late stage.
"I'd like to apologise to the people of Luton for the disruption and our members certainly didn't want to cause them disruption but they feel an injustice has been done and rightly so they feel they've taken the only action that is left to them."
The Royal Mail said there are 50,500 addresses in the area and about 150,000 letters a day are handled at the Luton office.