The RNIB is to transfer 438 employees to Action for Blind People in September, but trade union Unison said staff have been angered by the proposals.
The transfer involves every RNIB member of staff in Scotland, Northern Ireland and Wales, as well as a small number in England, the union said.
ABP is a wholly owned subsidiary of RNIB. It is registered as a charity in Scotland, but Unison says that in practice it does not currently operate north of the border, and will not be able to effectively support service users in Scotland.
Unison said the proposals were a “serious concern” and said that staff and service users did not understand why the changes were taking place.
Simon Watson, a national officer at Unison, said: “The major changes at RNIB are a serious concern. Staff have been left in the dark and very little has been done to reassure them. Staff are angry and concerned about these proposals.
“The fact that the proposal came with no real explanation means staff and service users feel very uncertain over the long-term future of the service and their jobs. There is also a risk the union may not be able to properly protect staff after the transfer. RNIB is yet to give a convincing explanation for such huge changes.”
However an RNIB spokesman said that the charity had conducted "an extensive period of consultation" including meetings with staff and unions.
"This is a disappointing story to see, causing concern among people with sight loss in Scotland unnecessarily," the spokesman said. "This work remains a continuing dialogue with staff and trustees."
He said that services and staff numbers would be unaffected by the transfer.
"We want to reach thousands more blind and partially sighted people that we aren’t reaching at the moment and we believe this change will help us to deliver this," he said.
"By sharing best practice from across the four UK nations, all working to the same standards, within the same structure, under the same direction of a single board of trustees we can make the most of our precious resources to do more for more people."
Since 2009 ABP and RNIB have operated a group structure, with merged frontline services. Last year the two organisations also announced the intention to merge back office services. However ABP retains its own chief executive and trustees.
Last year the group launched a new strategy, saying it wanted to reach “thousands more blind and partially sighted people”.
