The Liberal Democrats want the coalition to raise the income tax threshold above £10,500 by 2015, which could effectively mean a £100-a-year tax cut to 24 million basic rate payers.
The move, which would take half a million people out of income tax altogether, would represent a £1 billion tax giveaway before the next general election.
The Lib Dems have already seen the coalition achieve their manifesto commitment to raise the personal allowance to £10,000 - which was finally reached in the last Budget in March.
Lib Dem leader Nick Clegg will write to party activists next week calling for a "workers' bonus" to reward voters for the financial sacrifices they have made during the years of austerity.
More top news
The Novichok incident in Salisbury has turned into an international diplomatic crisis. But how bad could things get? Adam Shaw reports.
Among those murdered was Keith Palmer, an unarmed constable who "did his job and never wanted any fuss."
Top temperature 14 Celsius (57 F).