The chair of the Children's Hospital of Wales charity has said he knew nothing of Labour's £2m funding announcement until he heard reports on BBC Wales.
The hospital at the University Hospital of Wales in Cardiff
Lyn Jones said the revelation, made in a letter from Labour's Dr Brian Gibbons to the Western Mail two days before assembly polls open, was "political".
Dr Gibbons said the funding decision was taken in March but not publicised to avoid charges of electioneering.
The three opposition political parties have criticised Dr Gibbons' timing.
Speaking on BBC Radio Wales, Mr Jones said the Cardiff and Vale NHS Trust, which handles all the finances for the children's hospital, had also been unaware of the extra funding until they saw the letter.
At a meeting with ministers six weeks previously, Mr Jones said it was agreed that the Noah's Ark appeal, set up to raise funds for the children's hospital, would come up with details of running costs.
But there had been no indication that £2m was being allocated to the fund in that meeting, he said.
"This is not the way to deal with people. It doesn't improve the situation whatsoever," said Mr Jones, who is chair of the trustees at the Noah's Ark Appeal.
"I'm sure it's all political really. It's a shame it's all come out like this. We shouldn't have been dragged into it all."
Speaking on the BBC's Good Evening Wales programme, Dr Gibbons admitted he personally did not inform the Cardiff and Vale NHS Trust that the money was coming.
He said he believed the information would have got into the public domain had any communication of the extra funding been made to them.
The decision on the £2m was taken at the end of March, as one of Dr Gibbon's last acts as health minister before Thursday's assembly poll was called, and as a result of a business case submitted by Cardiff and The Vale NHS Trust.
But he said Labour felt they had to respond to editorial in the Western Mail published in April which criticised the hospital's £5m funding gap.
Dr Gibbons wrote in Tuesday's Western Mail: "At the end of March I decided not to make this decision public, precisely because I was convinced that, to do so just on the cusp of the election campaign, would lead to the sort of charges of claiming political advantage in which... this project ought not to become embroiled."
In the wake of the row, the Liberal Democrats complained to Wales' top civil servant Sir Jon Shortridge, asking whether Dr Gibbons had breached ministerial regulations by revealing the extra funding during the campaign.
His reply said it was a matter for Welsh Labour leader Rhodri Morgan to police the regulations.
Lib Dem leader Mike German subsequently said he would write to Mr Morgan asking for a statement on whether he believes Dr Gibbons' letter had breached the code.
Jenny Randerson, health spokeswoman for the Lib Dems said the money should have been announced before the end of the assembly term.
"The fact that neither the Noah's Ark appeal nor the Trust have been informed is, to be honest, quite astonishing.
"I think it is an electioneering move which has backfired."
Conservative health spokesman, Jonathan Morgan said Dr Gibbons' explanation was not credible and he should have announced the funding before the start of the official election campaign.
"Why leave it until these issues were going to be raised by the media?" he asked.
"This issue is being used by the Labour Party, by Brian Gibbons, in order to try and bolster what is left of their support on Thursday."
Helen Mary Jones, Plaid Cymru's health spokeswoman said the action amounted to "absolutely staggering incompetence".
"It is blatant electioneering," she said. "If the money was there a few months ago we should have been told."
The first phase of the £21m specialist unit for sick children was opened last year by Welsh Hollywood star Catherine Zeta Jones, patron of the public fundraising appeal.
The second phase - once built - will house an intensive care unit, operating theatres and surgical wards, at the University Hospital of Wales in Cardiff.