The current system is criticised for being unfair
Labour peer Lord Lipsey has attacked plans for free personal care for the frailest pensioners as a "gimmick" and urged ministers to reconsider the move.
The peer, a former member of the Royal Commission on long-term care, said he believed Gordon Brown was pursuing the policy for party political reasons.
The measure, outlined in the Queen's Speech, will provide help at home for about 400,000 people in England.
Mr Brown defended the policy as a "major breakthrough" in care provision.
He has admitted the current system of care for the elderly is not acceptable and that improving it will be Labour's "first domestic priority" should it win the next election.
The proposal to end means testing for care at home for the most vulnerable, including those with dementia, is seen as a "first step" to creating a national care service across England.
The government set out its ambitions for this in a green paper earlier this year but Lord Lipsey, a former adviser to Labour prime minister Jim Callaghan, said Mr Brown has now bypassed this process.
While it was right that ministers should look at ways of preventing pensioners having to use their savings or sell their homes to pay for care, he questioned whether it was better for people to receive care at home rather than in residential homes.
By ploughing ahead with his own plans, Mr Brown had performed "one of the most disorderly pieces of government I have ever seen in 40 years of political life", the peer told Radio 4's World At One.
"What has gone wrong is that in the middle on the consultation... the prime minister has declared that it is government policy that people with severe care needs in their own home should be paid in full.
"I think it is a bad policy but also a very bad way to do policy just to find a nice highlight for your Labour Party conference speech."
A more considered approach was needed to maximise funding for care from existing NHS and social services budgets and to ensure people could access advice and support more easily, he added.
"I am afraid that what has happened is that into this very complicated but important policy process has been injected something which is a bit of gimmick.
"I hope Gordon Brown will reflect on whether this proposal is in the national interest or merely designed to achieve party political interests."
The prime minister said Labour was committed to ensuring more people could retain their independence and dignity in old age by being able to stay at home and get help with cooking, cleaning and washing.
He told ITV1's This Morning: "It is a major breakthrough because for the first time people of whatever income... we have said we will help you stay in your own home and not have to go into institutional care."
The Tories have warned that the government may cut some disability benefits to fund extended care provision and have pledged to fight this.
"Labour has chosen to penalise one of the most vulnerable groups in our society for the sake of another eye catching announcement," sais shadow pensions secretary Theresa May.
But Health Secretary Andy Burnham described these suggestions as "disgraceful" and said the £670m required to finance the plan would come from his existing budget as resources are re-prioritised.
"I just think it is really gutter politics to raise concerns among some of the most vulnerable people in our society that their benefits will be cut," he told the BBC.
Mr Burnham said it was an option to cut attendance allowances - tax-free benefits for pensioners who need personal care because they are physically or mentally disabled - to fund long-term reform of social care.
But he said this had yet to decided upon, adding: "We have said that anyone in receipt of those benefits would get an equivalent level of support."