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
A Met Office warning covering much of England and south-east Wales is expected to remain in place until 11am.
A band of cloud and patchy rain is expected across Scotland and Northern Ireland, moving southeast as the day progresses.
Charity warns an estimated 30 child refugees a day are being expelled from EU countries after crossing the border from Serbia.