The ILM has announced today that Simon West, its chief executive, will leave the charity in September for health reasons.
It said that West’s departure had "prompted each member of the board to consider their own future involvement with the ILM in a leadership capacity", and resulted in Helen Hoare, its chair, and board member Andrew Sawbridge deciding to step down from the board.
"For some time now Simon has been battling a chronic condition, and he has made the difficult decision to leave the ILM on 19 September to concentrate on his health and recovery," a statement from the ILM said.
"While both organisations remain firmly in favour of the merger, we have agreed that the merger plans should be put on hold while decisions are made about how to replace Simon."
Maria King, who will remain on the board with three others, will become interim chair.
"It has been a privilege to be a director of ILM for the past six years, but after much reflection I have decided that now is the right time for me to stand down as chair and from the board at the end of September," said Hoare.
"I will be around until then to provide continuity and provide support to Maria as she forms a new board to take ILM forward from this point."
The merger proposal, which was first announced in April, would involve the ILM being dissolved as a company and its three members of staff joining the IoF, which employs 40 people.
The ILM would become a special interest group of the IoF, which collectively elect one person to join the IoF board. The ILM branding would be maintained under the new SIG.
But the proposal was opposed by some members who said they felt it was not in the best interests of the ILM and accused the ILM board of trying to rush it through.
The ILM said it would delay the merger for further consultation and Hoare subsequently said that the board would consider resigning if the merger proposal was rejected by members.