Voters have delivered a nasty blow to the Turnbull government just as it embarks on a last-ditch effort to pass company tax cuts.

In Sunday night’s Newspoll, Labor scored its highest primary vote in three years, at 39 per cent to the Coalition’s 37 per cent, making the Prime Minister’s 29th straight poll loss since replacing Tony Abbott all the more galling.

The government has made company tax cuts its main policy push of recent weeks, as it races the clock to pass the measure in the last parliamentary sitting week before the May budget.

But the voting public has not responded warmly, with Labor leader Bill Shorten gaining one point for voter satisfaction to put him ahead at 34 per cent compared to Mr Turnbull’s 32 per cent.

Earlier on Sunday, before the poll results were known, the government’s chief negotiator in the Senate, Finance Minister Mathias Cormann, ramped up pressure on the two senators yet to commit to the tax plan – Derryn Hinch and Tim Storer.

“If the Senate were not to pass our business tax cuts in full, it would be a deliberate move to put our businesses here in Australia at a competitive disadvantage,” Senator Cormann said.

It comes as Labor calls for a Senate inquiry into a pledge from the Business Council of Australia (BCA) to invest more in Australia if the corporate rate is slashed.

The government now has seven of the nine crossbench votes required for the plan to pass after Pauline Hanson’s backflip last Thursday.

But after this week, Parliament will not sit again until May, meaning the legislation must pass before Wednesday for the tax cuts to be factored into the budget.

The government’s tax plan would reduce business taxes from 30 per cent to 25 per cent for all businesses at a cost to the budget of $65 billion over 10 years.

Labor, which remains staunchly opposed to the tax cuts, will call for a Senate inquiry on Monday into the commitment from the BCA and companies including BHP, Fortescue Metals and Qantas.

The BCA letter followed calls from Senator Hanson and Senator Hinch for business to commit to use the extra revenue to create jobs and lift wages.

Labor’s inquiry would report the day before the budget and would investigate the specifics of any investment plans that the companies claim will result from a reduced corporate tax rate.

It would also examine the level of tax paid by the companies over the past five years and investigate how much the companies would expect to reap from the lower rate.

The charity and social services sector also entered the fray at the weekend, arguing the tax cuts would mean “further budget cuts are inevitable”.

“We are concerned that already disadvantaged Australians may pay more for health, education and community services,” organisations including Anglicare, Oxfam Australia, the Salvation Army and the Australian Council of Social Service wrote in a letter to the crossbench.

The opposition is remaining tightlipped about its approach if the tax cuts for all businesses with a turnover up to $50 million are passed. That tranche of cuts is worth about $36 billion to the budget.

Asked if a future Labor government might repeal the cuts, shadow employment spokesman Brendan O’Connor said: “Let’s just see.”

“Clearly Malcolm Turnbull has a view that it should be providing it to the big end of town, and he has such a strong faith in market forces he believes that somehow, through osmosis, we will see wage increases,” Mr O’Connor told the ABC’s Insiders on Sunday.

“That’s not a view held by many economists and certainly not by Labor.”

Senator Hinch has suggested he remains sceptical about the tax plan, though he is said to be seeking funding boosts to housing affordability measures and to help pensioners as political cover before he could support the legislation.

His proposal last week to exclude the banks from the tax cuts was swiftly rejected by the government.

Meanwhile, Senator Storer, who was sworn in just last week, has declined media requests, with his office saying only that he is consulting widely as he considers his position.