The Coalition government secretly proposed a $660 million boost to welfare payments in its failed efforts to secure Senate crossbench support for tax cuts for large companies, even offering senators the opportunity to claim credit for the rare increase.

The proposal, which would have introduced a $5 a week boost to rent assistance payments for welfare recipients, shows how much the government was willing to offer to pass the second stage of its centrepiece company tax cuts for businesses with turnovers greater than $50 million.

But the welfare boost, which Fairfax Media understands was made to multiple crossbenchers around March and costed at $660 million over four years, falls way short of the increase demanded by the Australian Council of Social Service, the Greens and crossbench senator Tim Storer.

The Senate rejected the government's tax cuts for large companies despite a series of sweeteners. Credit:Alex Ellinghausen

A modest increase to welfare payments was just one of the sweeteners offered by the government in its doomed push to pass tax cuts for large companies before dropping the measure in August as Malcolm Turnbull fought to survive as prime minister. In a separate last-ditch effort on tax cuts, the government had offered to exclude banks.