Gordon Brown's authority is tested further today with the powerful Commons Treasury Committee launching its own carefully worded attack against the UK Government for penalising the poor, claiming the abolition of the 10p income tax rate is "unreasonable".

Coming on the back of unease on Labour back benches over the tax change, coupled with gathering economic gloom and the Tories regularly hitting over 40% in the polls - one yesterday put them 11 points clear on 43% - some Westminster-watchers feel a watershed in the life of the 11-year-old Labour government has now been reached.

Sensing this, David Cameron sought to pile the political pressure on the Prime Minister by urging Labour MPs to step up their rebellion against the scrapping of the 10p rate.

"Every Labour MP should be thinking how can we help the Conservatives and try and stop these tax increases going through at a time when people are finding it very tough," he said.

While some MPs have been complaining for months about the abolition of the 10p rate, which takes effect today, the consequences of the credit crunch with the prospect of more UK homes being repossessed has brought their concerns to the top of the political agenda.

Already dozens of Labour back benchers have made their concerns public in Commons motions and today they are joined by Westminster's Labour-dominated Treasury Committee.

As it branded the raising of the tax level for the low paid "unreasonable", John McFall, its chairman, said: "While tax simplification is a laudable aim, it seems strange that the abolition of the 10p starting rate of income tax disadvantages mainly low-income households.

"As such, the government must ensure that these people are identified and appropriate help given to them to ensure they receive the benefits to which they are entitled," added the MP for West Dunbartonshire.

George Osborne, the Shadow Chancellor, seized on the committee's report, claiming: "Labour's tax revolt is growing."

However, the strongest criticism came from LibDem leader Nick Clegg who spoke of British politics reaching a "defining moment", claiming Labour no longer had the right to talk about championing fairness in society.

He told the BBC the UK Government was "abandoning" more than five million low-paid people "so Gordon Brown could offer a 2p cut in the rate from 22p to 20p for the following day's headlines". Mr Clegg added: "That's an outrageous political stunt at the cost of the poorest in British society."

However, John Hutton, Westminster's Business Secretary, made clear there would be no going back on abolishing the 10p rate, noting how it had already been voted on.

He claimed the tax change had been part of a "balanced package", which had left families with children "significantly better off" and that Labour's record should be seen over a longer period.

"Since 1997, low-income families have gained very significantly from tax credits and tax changes that the Labour government has been able to introduce - something like nearly £10-a-week better off."

Acknowledging there were concerns over the scrapping of 10p rate, Mr Hutton added: "They have been raised with ministers. Obviously, it is right and proper ministers engage in a debate with our concerned colleagues about this."

In Scotland, the SNP sought to draw Wendy Alexander, Labour's leader at Holyrood, into the row. Claiming that 324,000 low-paid Scots would be hit by the abolition of the 10p rate, Nationalist MSP Kenneth Gibson said: "Scottish Labour's silence on this tax hike is deafening."

Last night, Labour hit back, accusing the SNP of hypocrisy. "If they cared at all about hard-working families, they would abandon their plans for a local income tax that will see families facing 25% hikes in their bills," a party spokeswoman told The Herald.

She added 237,000 families in Scotland would benefit from increases in Working Tax Credit and more than 49,000 lone parents would get an extra £40 a week when they got into work.

As yesterday's ICM poll also showed 59% of people were cutting back because of the impact of the credit crunch, Mr Cameron pointed out how about 1.4m mortgage-holders were going to see monthly payments rise by about £200.

Elsewhere, Labour loyalist George Mudie urged the UK Government to strike a deal with lenders to delay home repossessions.

The Leeds MP said ministers should "try to do everything to keep people in their houses", adding: "I don't think the government has actually grasped that."

As Mr Brown comes under increasing pressure on the economy, observers suggest there are signs discipline is beginning to crack.

Already Ivan Lewis, Westminster's Health Minister, has admitted the UK Government was "losing touch" with voters, and Gerry Sutcliffe, the Licensing Minister, backed the drinks industry in attacking the Budget rise in alcohol duty, only for him to later retract his comments, claiming they "did not reflect my views".