David Cameron dismissed claims families with children will lose £511 a year on average from today under tax and benefit changes - insisting the Tories were the party of "the strivers, the grafters, the family raisers".

The Prime Minister said he did not accept the figures produced by a respected think-tank which sparked warnings from anti-poverty campaigners that the start of Easter weekend would be "Bad Friday" for a million or more households.

And he took to the local election trail promising a "flat-out, full-throttle fight" in an effort to wrest back the political initiative after a bruising fortnight for his leadership across a range of issues.

Campaigning in Wales ahead of a key test of voter opinion at the ballot box across most of the UK on May 3, the latest challenge came from figures compiled by the Institute for Fiscal Studies (IFS) on the impact of cuts to tax credits.

They showed families with children bearing the brunt of a number of tax and benefit measures coming into force from the start of the financial year.

Shadow chancellor Ed Balls said it was a "tax credits bombshell" for a million households that would leave some couples better off on benefits than in work.

Cameron acknowledged "difficult decisions" had been taken on tax credits but said 24 million taxpayers would be £6.50 per week better off due to a rise in the income tax allowance.

He said he "did not accept the figures" from the IFS.

In a speech to activists in Llandudno, he urged them to tell voters that the Tories were the party "that cares about the strivers, the grafters, the family-raisers, the community-builders".

The premier dismissed mounting criticism of the Government and declared ministers were "looking at the horizon not the headlines".

His leadership is under intense scrutiny after a tumultuous fortnight for the coalition - and the Tories in particular - which has seen his personal poll rating slump to a new low.

Yesterday he was forced to defend anti-terror plans for secret court hearings and e-mail monitoring amid open criticism from Tory backbenchers and Liberal Democrats including Nick Clegg.

The moves were the latest focus of a sustained run of negative comment about badly-received policies and the handling of issues such as the threatened fuel tanker drivers strike.

"We may take our hits but always remember this: we are making the hard, long-term decisions that are vital to the future of Britain," he said.

"We are doing our duty by our country and that has always been the Conservative way."

He said he would ignore "whispering voices" telling him to hold back on radical changes.

"Yes, cutting spending is tough. But in the years ahead people will look back and see that what we're doing today has laid the foundations for a sounder, stronger economy."

He went on: "We need a flat-out, full-throttle fight.

"That's what we need to tell people: that this is a government that's looking at the horizon, not at the headlines, that cares about working for the long-term good, not short-term popularity, that works in the national interest, not the party interest."

Amid signs Labour is not capitalising on the difficulties, including its shock defeat to Respect's George Galloway in the Bradford West by-election, Cameron launched a withering attack on the Labour-run Welsh Government.

It was presiding over "a complete and utter shambles" in the NHS and letting pupils fall behind English colleagues by "clinging on to old, failed, stale ideologies" in education, he said.

"While all this is happening, Welsh children slip further and further behind in the international rankings," he said.

"Against other developed countries they come 30th for science, 38th for reading, 40th for maths - below average across the board - unlike any of the other nations in the UK.

"You'd think this called for some real soul-searching, some real ambition, but you know what the Welsh Education Minister said? That he didn't expect any "real improvement" by the next rankings - a whole three years later.

"This is nothing less than a national scandal.

"It might be the political culture in Wales just to go along with the status quo and not rock the boat but we've got to be bolder than that."

Welsh Labour described the PM's comments as "offensive smears".

Torfaen Assembly Member Lynne Neagle said: "Unlike the Tories in England, Welsh Labour in Government has worked with the teaching professions to implement successful change.

"That's why we don't have league tables. That's why we've abolished SATs, introduced the Foundation Phase and created a skill-based curriculum.

"We oppose the Tory obsession for a model of education that's driven by antagonism and competition. It's why we've rejected
academies and so-called free schools.

"Welsh Labour has maintained faith in the comprehensive model of education because it delivers for all our children.

"That's what the people of Wales voted for in the Assembly elections last May. We have a mandate from the people for our policies - David Cameron doesn't."