In an unusual show of support for Royal Mail cuts, union leaders have publicly endorsed management plans to phase out bikes for postal deliveries.
The CWU said it "fully supported" the changes, which will see the current fleet of 24,000 bikes reduced to just a few hundred.
The union said thousands of postal workers have been injured in cycling accidents.
The plan has been attacked by cycling campaigners.
Under the slogan "keep posties cycling", they cycled to Royal Mail's London headquarters on Wednesday to deliver hundreds of letters of protest.
Kevin Mayne, chief executive of the national cyclists group, CTC, said: "Postal worker cyclists and members of the public have expressed concern at the illogical and rash way Royal Mail has made this decision."
But the CWU accused CTS of misrepresenting the facts.
"For every postman and woman who loves their bicycle, there's one that hates them," a union spokesperson said.
"We fully support the changes for improved efficiency, safety and environmental performance. Most important is the modernisation of the industry.
"Today's protest by CTC used ancient cycles and old postal uniforms. We're fighting for a modern Royal Mail. You have to let go of some of the sentimentality surrounding traditional views of an industry spanning hundreds of years if you want to have a successful future."
The union said that 13 cycle delivery postmen and women had been killed at work over the past 15 years, and thousands more injured as the result of road traffic accidents.
The CWU said the decision to cut use of bikes was taken two years ago as part of the plans to modernise deliveries.
A Royal Mail spokesman said: "We are not getting rid of all bicycles and they will remain part of our delivery operation, with decisions taken locally."