A name for the new organisation has not yet been chosen and a chief executive has not yet been appointed. The two existing chief executives have not yet said whether they will apply.
A spokeswoman for the Princess Royal Trust said that redundancies were a possibility but had not yet been confirmed. Nor had it been decided, she said, whether the combined charity would retain both existing offices or move to new premises.
Crossroads Care employs 41 people, supports 96 charities and has a turnover of £3.76m. The Princess Royal Trust employs 70 people, supports 142 charities and has a turnover of £5.03m.
The charities expect to continue providing all their existing support services, the spokeswoman said. Charities that form part of their networks will not be asked to merge, change their names or change their activities as a result of the umbrella bodies merging, she said.
The two charities have already worked closely together on fundraising and policy, have jointly funded a number of posts, and have been collaborating on a merger for the past 18 months, the spokeswoman said.
The Princess Royal will serve as president of the new charity, a role she has held at the trust since it was formed in 1991. Andrew Cozens, former vice-chair of the trust, will become chair of the new charity. The two existing chairs will remain on the board on an interim basis, and a final decision about the composition of the board will be taken later in the year.
Cozens said: "The work undertaken by Crossroads Care and the Princess Royal Trust for Carers is complementary and they have a well-known and respected track record of working together.
"While both are doing great work independently in supporting carers, it became apparent that so much more could be achieved, both now and in the future, if the two came together to create a new charity."