The British Transport Police Federation described re-planning work currently being undertaken as a “hollow PR exercise dressed up as a rigorous review”.
The integration of BTP into Police Scotland had been due to take place in April next year, but was delayed earlier this year amid concerns over IT and pension arrangements.
The Scottish Government has come under sustained criticism over the plan which is opposed by staff associations, rail unions and opposition MSPs.
A joint programme board, which includes representatives of Police Scotland, the Scottish Police Authority and the Scottish and UK Governments, is overseeing the merger.
Nigel Goodband, chair of the BTP Federation, said: “As we approach the deadline for agreeing a new date for integration, I have grave concerns ministers will be misled as to the progress made during the re-planning phase and the revised date they will be invited to consider won’t in any way be evidence-based.
“The re-plan is a hollow PR exercise dressed up as a rigorous review. This is integration at all costs, including that of public safety. The complexities are now obvious but the supposed benefits, costs and impact on the protection of the travelling public are no clearer than they were two years ago.”
In April, the start date for the integration was postponed after Deputy Chief Constable Iain Livingstone, of Police Scotland said it had become clear the plans could not be put in place by the deadline without “compromising public safety”.
MSPs voted to integrate BTP operations in Scotland into the national police force last year despite concerns.
The financial memorandum which accompanied the Bill said there would be “minor transitional costs”, however it later emerged there would be “significant financial implications”.
Dr Kath Murray, a policing researcher at Edinburgh University, said the problems stemmed from a failure to establish the risks and costs before the Act passed.
She said: “Given the legislation has not yet commenced, this situation can be easily resolved through a commissioned service model, or similar, that realises the same benefits, avoids passing on unquantified risks and liabilities to the SPA, and is much more likely to secure the support of experienced BTP officers and staff.”
A Scottish Government spokesman said: “Significant work has been done with partners including Police Scotland, BTP and the BTP Authority to establish a realistic date for integration. This robust work will be subject to rigorous independent review, and we will continue to work closely all partners to address any issues that emerge.”
Tom McMahon, of Police Scotland, said: “The priority for Police Scotland is to ensure that we continue to deliver the highest possible standards of service and that all of our communities, including people who work on and use the rail network, are kept safe.”