Home Secretary David Blunkett has asked to see the book ahead of its publication but it is thought unlikely he will take legal action to block it.
Steve Moxon's The Great Immigration Scandal is expected to allege failings in Mr Blunkett's immigration policy.
The 48-year-old lost his job after claiming migrant checks were waived.
His revelations about the Sheffield immigration office led, in part, to the resignation of Home Office Minister Beverley Hughes.
'Reasonable'
The Home Office says civil servants, whether serving or retired, cannot publish information about their work without official permission.
A spokeswoman told BBC News Online: "Mr Moxon and his publisher have been asked to provide a manuscript of his book and have been assured that permission to publish will not be unreasonably withheld.
"The Home Office would consider its position should Mr Moxon or his publisher refuse to comply with the request. "
It is thought the situation would have to be very serious for Mr Blunkett to apply for an injunction to stop the book.
On the eve of publication, Mr Moxon said: "There's nothing in the book which the Home Office should have any problem with at all."
He said he would react "with glee" if the government did try to block publication.
"The one way the government effectively chucks sales through the roof of any book is to ban it," he added.
The book is due to be launched at 1700 BST at Blackwells bookshop in Sheffield. The initial print run is 2,000 copies.
Checks 'waived'
Mr Moxon, 48, first came to prominence when he made claims that key checks were waived by immigration staff in Sheffield to make the numbers coming to Britain seem less dramatic when the EU expanded in May.
He insisted he "exhausted all possible routes" before talking to the newspapers.
Mr Moxon, who was suspended immediately after he went to the press in March, was told last month he would be sacked for breaking the terms of his contract.
He intends to take the government to an employment tribunal, claiming his actions are covered by the Public Interest Disclosure Act, which allows workers to expose wrongdoing at work.
An extract of his book published in the Sunday Times described his frustration at what he said was Home Office stonewalling when he tried to raise concerns about abuses of the immigration system.
Mr Moxon attempted to e-mail his superiors about what he saw as "a progressive institutional failure to apply the rules" but got no response.
He wrote: "The attitude to applications for residency at the department was that if it arrives on your desk then you should grant it, if at all possible, because if you don't the applicant will disappear and stay in the country illegally."