Sir Martyn Lewis, outgoing chair of the National Council for Voluntary Organisations, has called for the organisation to create a "direct rival" to Acevo after claiming the charity leaders body had repeatedly rebuffed attempts to merge.
Lewis, who was speaking at the NCVO's trustee conference in London yesterday, his last public event before stepping down as chair in two weeks' time, said a merger between the two umbrella bodies was a "sensible and necessary move" to reduce duplication within the sector.
But he said he believed it would never happen, blaming Acevo's lack of cooperation and unwillingness even to explore whether a merger was the right option.
"Looking back I can now safely say that talking with Acevo has been a little bit like juggling with sand," he said. "One day you see possibilities on the distant horizon, a week later you realise it was only a mirage."
Acevo later said his views "did not reflect the conversations we have had in private on a range of issues".
Lewis said in his speech that during his six years as chair of the NCVO he and Acevo’s chairs had always had a will to talk, but that "somewhere in the heart of Acevo, whether at board level or elsewhere, any hint of cooperation that might eventually lead to a merger has been repeatedly pushed away – without exception".
He said: "After six years of this I have come to the conclusion that this sensible and necessary merger will never happen, despite growing pressures from the sector.
"And if I'm right about that, my proposed solution is this: that NCVO itself sets up a new division specifically to look after the interests of charity chief executives – a direct rival to Acevo."
He said his comments stemmed from a "real sadness" rather than anger and had not been discussed with any of his colleagues at the NCVO.
He said many of his colleagues at the NCVO, including Sir Stuart Etherington, its chief executive, "may well throw up their hands in horror at such a confrontational suggestion, but as outgoing chairman I would at least like to lay that on the table as a way forward, dictated by six years of experience and frustration". Lewis added that he would be interested to hear sector feedback.
The two bodies had collaborated successfully last year in responding to attacks by national newspapers on high-profile charities, and had engaged in a series of further meetings earlier this year after Sir Stephen Bubb, who was then Acevo's chief executive, announced he would be stepping down, said Lewis.
He said this would have provided an ideal time for a merger, but this hope had been dashed when Acevo advertised for a replacement chief executive and two other senior executives.
He said several influential charity leaders had approached the NCVO and Acevo to express support for a merger and that in a context where charities were also being asked to fund the Fundraising Regulator, many were unwilling to fund two umbrella bodies.
He said the charity sector as a whole needed to tackle duplication in order to signal to potential donors that it was "getting its act together".
Lewis also expressed disappointment that very few charities had taken up the recommendations made by his 2014 inquiry into charity chief executive pay, which called for all charities to display how much they paid their leaders and why in easy-to-find parts of their websites.
In a statement, Acevo said the opinions expressed by Lewis did not reflect the conversations it had had in private on a range of issues.
It said: "We intend to respect the confidentiality of those conversations and to continue them in a constructive spirit."
The statement said Lewis was a "hugely respected advocate for the charity sector" and Acevo would be sorry to see him go.
It said Acevo would continue to work with the NCVO on issues of sector-wide importance, including recent joint letters to the government on anti-advocacy clauses and proposals to the Chancellor regarding the Autumn Statement, and would continue to have "constructive conversations on a range of issues".