Royal Mail says productivity in some offices is 10% worse than the average.
Thousands of postal workers in London are to strike.
Up to 10,000 Communication Workers Union (CWU) members will walk out for 24 hours on 19 June causing disruption to mail deliveries across the city.
They claim "arbitrary" job cuts will affect about 1,600 workers and that staff may be downgraded into part-time positions, threatening services.
The Royal Mail disputes the job cut figure and said a fall in mail volumes meant fewer jobs were needed.
CWU deputy general secretary Dave Ward said: "Royal Mail is blocking modernisation by refusing to negotiate change with the CWU.
"We have offered a moratorium on all strike action if Royal Mail will suspend executive action and enter into meaningful negotiations."
Mr Ward added: "The future of the business must be safeguarded through careful planning, not shooting from the hip."
But a Royal Mail spokesman said: "A strike will not modernise Royal Mail - it will simply disrupt the service to which customers are entitled, lead to an even greater loss of business and leave Royal Mail far less able to protect full time jobs.
"Productivity in Royal Mail offices in London already lags behind the rest of the UK with the productivity in parts of London now 10% worse than the UK average."
He added: "We are only putting in place changes which are already agreed with the CWU as part of the 2007 deal."