LONDON (Bloomberg) - Chancellor of the Exchequer Alistair Darling yesterday defended a tax increase for 5.3 million of the UK's lowest earners in the face of declining poll ratings and mounting opposition from lawmakers in the ruling Labour Party.

The Finance Minister led a debate in the House of Commons on the Finance Bill, which enacts measures in last month's budget. The Treasury eliminated the 10 percent bracket to fund a cut in the basic rate of income tax to 20 percent, from 22 percent, starting this month.

"It's completely at odds with everything else we've done since coming into office in 1997," Des Turner, a Labour member of Parliament, said in an interview. "We're gravely disappointed and worried by this. It's a mistake, and we hope that some way can be found to redress it."

More than 70 of the 351 lawmakers in Prime Minister Gordon Brown's Labour Party have expressed concern about the changes. The unrest in Labour's ranks also reflects increasing disgruntlement with Mr. Brown's leadership and infighting among lawmakers as the government's poll ratings slide.

Backing for Labour fell three points to 30 percent in the last month, a Populus Ltd. poll published on Sunday showed. That is Labour's lowest in any Populus poll since Mr. Brown took office in June. The Conservatives had support of 40 percent.

Many workers in Britain will first feel the impact of the changes when they receive their monthly paycheck on April 30, one day before voters in England and Wales go to the polls to select a new set of local councillors.

Ian Gibson, one of the lawmakers opposing the tax change, said Mr. Brown may soon face a "poll tax moment," a comparison to the revolt among members of Parliament that led to the ousting of Prime Minister Margaret Thatcher in 1990.

Charles Clarke, a former Home Secretary, added his voice to the dissenters today, calling the tax change "disappointing". He also became the second senior Labour lawmaker in less than a month to lock horns with Ed Balls, the education secretary who is one of Mr. Brown's closest allies.

Replying to Mr. Ball's suggestion that lawmakers should not criticise the prime minister anonymously, Mr. Clarke said in a letter to The Times that Mr. Balls often used such off-the-record briefings in the past.

"I would advise him to examine himself and his own role," Mr. Clarke wrote. "He should stop attacking others anonymously or in code and look to his own record."

The Conservatives and Liberal Democrats, the two main opposition parties, joined Labour lawmakers led by former pensions minister Frank Field in calling for the change to be reversed.

"I cannot think of another recent example when a tax change has targeted a rise on some of the lowest paid people in the country," Conservative finance spokesman George Osborn said yesterday on BBC Radio 4. "At the very moment when those families are feeling the squeeze, they're being hit with a tax rise."

If joined by all 70 Labour lawmakers, the opposition parities will have enough strength to defeat the government on the measure. Mr. Darling vowed yesterday to keep the proposal for now, saying he would revisit the decision in the pre-budget report due in the final months of this year.