NFU Scotland has urged a Scottish parliamentary committee to hold an extraordinary session on the issue.
The union said it did not believe payouts under the new basic payment scheme would be delivered in the normal December window.
The Scottish government said it continued "to work flat out" towards starting payments by the end of 2015.
The basic payment scheme is the main EU subsidy available to farmers in Scotland, and is allocated by the Scottish government following Common Agricultural Policy (CAP) reforms.
NFU Scotland said that "despite Scottish government statements to the contrary", it did not believe there was "any likelihood" of payments being delivered in December.
It now wants Rural Affairs Secretary Richard Lochhead to make a public statement on when farmers will receive their payments and what their value will be.
The union has also written to the Rural Affairs, Climate Change and Environment Committee, asking it to hold an extraordinary session to establish the anticipated timeline for payments.
NFU Scotland president Allan Bowie, said: "Whilst the cabinet secretary has sought to allay fears that the payment schedule will slip, Scottish government's message has shifted from payments being delivered in 'early December' to 'late December', and we are now convinced that these payments will not be made until 2016.
"We also note with interest that... the Rural Payments Agency gave an unambiguous statement... that it will commence paying the new basic payment scheme in full to farmers in England in December, with the majority receiving payment by the end of that month and the vast majority of payments completed by the end of January.
"Scottish farmers and crofters - and all those who provide goods and services to our sectors - would benefit hugely from a similar clear statement on payment timetables for Scotland to allow them to plan their businesses accordingly."
Meanwhile, Mr Lochhead announced that farmers and other land managers would begin to receive their share of Â£6.7m of rural development payments from next week.
He said the Scottish government's Rural Payment and Inspectorate Division had begun processing more than 5,000 Land Managers Option payments covering 2014.
The scheme supports agri-environment and animal health and welfare measures.
The vast majority of LMO payments are expected to be paid within two weeks.
Mr Lochhead said: "Today's announcement demonstrates how the Scottish government is delivering for Scotland's farmers as we deal with gargantuan challenge of implementing the biggest CAP reform in a generation.
"We continue to work flat out to be able to begin making basic payments by the end of the year."

Labour's Leigh Bramall said he was leaving the council to take up a job at a communication company in Sheffield.
He said he remained "100% committed" to the work of the Labour run-authority but added "I have to put my family first".
Council leader Julie Dore paid tribute to Mr Bramall's work saying he had "given his all to the job".
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Mr Bramall, 42, has represented Southey since 2004 and was appointed deputy leader of the council in 2015.
He said: "I have put everything into my job and love what I do.
"But I have to put my family first and now is the right time for me to move on.
"I am moving to a fantastic Sheffield company and feel this is a great opportunity for me to start a new chapter in my professional career which also allows me to stay in Sheffield, the city that I love."
Ms Dore said: "He has been dedicated to serving the people of Sheffield as Deputy Leader of the Council and given his all to the job.
"This inevitably takes its toll on your personal life and I completely understand and respect the decision he has made. I wish him all the best for his future career."
Pressure is mounting on the Scottish government to confirm when EU support payments will be delivered to farmers.
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The deputy leader of Sheffield City Council has announced he is to stand down after 13 years as a councillor.