Millions of couples face higher income tax under Labour's plans to axe the marriage allowance, the Tories have claimed.
Jeremy Corbyn has promised that people on less than £80,000 will not pay more if he wins on June 8.
But Conservatives insisted the Labour leader's plans to scrap the tax break for married couples, set out in the party's manifesto costings, leaves millions facing a loss of up to £230 a year.
More than four million couples are eligible for the relief and around 1.6 million have applied for it.
Stepping up the attacks as the General Election campaign enters its final days, Treasury minister David Gauke said a Labour victory would lead to the highest taxes faced in peacetime.
He said: "Jeremy Corbyn has already broken his promise on tax. Under his plans, taxes would rise for millions of ordinary working families across the country even before his shadow chancellor has had to pay for the black hole at the heart of their uncosted spending plans.
"Putting Jeremy Corbyn in Downing Street would mean the highest taxes this country has ever faced in peacetime, the Brexit negotiations at risk and our national security in the hands of Diane Abbott.
"The only way to keep taxes low and the economy strong is to vote Theresa May and the Conservatives on Thursday."
The marriage tax allowance was introduced under David Cameron and allows someone to transfer £1,150 of their tax-free personal allowance to a partner.