Earlier this month, a group calling itself Save Big Lottery claimed in a blog that ministers were planning to cut the BLF’s annual share of the proceeds from the National Lottery by 48 per cent on last year to make up for budget cuts at the Department for Culture, Media and Sport.
It is not known who is behind the blog and the Treasury has declined to comment on speculation before the review.
The BLF has also declined to comment on the possibility its budget will be slashed.
A parliamentary debate will take place in Westminster Hall this afternoon to discuss the possible effects of such a cut.
In a statement, Unite described the possible move as "the big robbery at the Big Lottery" and urged ministers to "come clean about their sleight of hand intentions".
It said projects such as young people's mental health interventions, women's refuge projects and food banks, which receive BLF funding, could be among those affected.
Unite national officer for the not-for-profit sector Sally Kosky accused Chancellor George Osborne of being a "Robin Hood in reverse... robbing from the poor to give tax breaks to the wealthy".
She said: "David Cameron has made much of his so-called big society vision, but here he is acting as an accomplice, heaping on misery to those that have been knocked sideways by savage cuts to working tax credits, unemployment and homelessness – the very people who seek assistance from charities.
"Charities are often the last resort for the vulnerable and they won’t be able to withstand the tsunami of cuts to services that a £320m loss of funding will entail."
Unite said it represented the majority of union members working for the BLF.
A BLF spokeswoman said: "We are unable to comment further at this time. Our funding reaches every community across the UK and supports people to build better lives for themselves, their families and communities, with over 13,000 grants made last year alone.
"Our priority remains focused on the interests of our thousands of grant-holders and the millions of people whose lives are affected by our work."
Unite is not the only group to have expressed concern over the possible cuts.
Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, has warned the move could mean the BLF "all but closes its doors to new funding applications".
The NCVO and the charity leaders’ body Acevo have written letters to senior government ministers warning of the profound impact the cut could have.
Ciaran Price, policy officer at the training and publishing charity the Directory of Social Change, called the prospect of the funding cut "outrageous", while Neil Cleeveley, chief executive of the local infrastructure body Navca , said it was "disgraceful".