The cost of moving Scottish Natural Heritage's headquarters from Edinburgh to Inverness could be almost double the original estimate, it has emerged.
The majority of staff are being moved to Inverness
Consultants have predicted that the bill could top £40m, prompting fresh calls for a rethink of the proposals.
But the estimate has been dismissed as "speculation" by SNH.
Meanwhile, a union is planning to ballot its members to see if they are prepared to strike over the proposals, which would affect more than 200 staff.
The plans to transfer jobs to Inverness are part of the Scottish Executive's efforts to spread public sector jobs around the country.
A report by consultants DTZ Pieda initially suggested that the move to the Highland capital would cost about £22m.
SNH later suggested that the bill would be closer to £30m.
Now a report by Turner and Townsend has estimated that relocation expenses, redundancy payments and a new office in the Highland capital could cost the taxpayer more than £40m.
SNH chief executive Ian Jardine said this prediction did not represent the most likely scenario.
"A lower estimate was more realistic, based on current knowledge of what SNH will need," he said.
"The higher figures quoted in the media all include the total capital costs of a new building. It is unlikely that SNH would purchase a new building but would seek to agree a rental cost over a number of years.
"The true costs will not be known until the various pieces of work have been
tendered and consultations with staff are complete.
"Given the work required to do that, these are not likely to emerge until around January."
The executive said it was standing by its policy of public sector job dispersal.
"Overall costs will depend on the detail of the
plan, but we do not recognise £40m as a realistic figure," said a spokesperson.
However, Edinburgh East Labour MSP Susan Deacon, a critic of the move, said it was further evidence that the decision to move staff against their will was flawed.
She has called on the executive to rethink the decision.
Liberal Democrat Mike Pringle, the MSP for Edinburgh South, said it was not certain that SNH would be able to rent an office in a "hotspot" like Inverness.
"If a new building is the option no-one
knows what the cost will be, or what developers will hold out for," he said.
"I'm not confident that the cost will remain at £30m, and my gut feeling is that it will rise considerably."
Scottish Tory leader David McLetchie, the MSP for Edinburgh Pentlands, said: "I am appalled that once again this government seems to care not one jot for taxpayers' money and how it is spent.
"It's bad enough that SNH staff are put through emotional turmoil and faced with family upheaval, but the squandering of tens of millions of pounds only adds insult to injury."
Fergus Ewing, the Scottish National Party MSP for Inverness East, Nairn and Lochaber, said he supported the move.
But he said: "This Labour policy has been worked out on the back of a fag packet.
"The rising costs of the SNH move are beginning to mirror the ever rising costs of that other Labour policy - the Holyrood project."
Meanwhile, the Public and Commercial Services (PCS) union said it hoped to organise an "indicative" ballot of SNH workers next week.
Albie O'Neill said: "We're planning to ask members if they would be prepared to
take industrial action, before we go to an authoritative ballot.
"Industrial action would be either to strike, or take selective disruption aimed at disrupting the passage of the Nature Conservancy Bill."