Amendments have already been made to the Coroners and Justice Bill
Proposals to allow some inquests to be held in private should be scrapped, says a committee of MPs and peers.
Inquests must be "transparent" -especially for deaths involving the state, such as police shootings, the joint committee on human rights said.
Ministers had offered "no proper justification" for the plans, it said.
The government argues that in some cases inquests should be held in private for national security, crime prevention or diplomatic reasons.
But it has tabled amendments to the Coroners and Justice Bill in which a wider "public interest" justification for holding an inquest in secret has been dropped.
The public interest clause had been criticised by the committee which said in its report it was "alarmed" that it had been included "'just in case' a future unforeseen concern might arise".
Justice Secretary Jack Straw also announced that the decision to hold an inquest in private would have to be made by a High Court judge, not ministers.
The committee's report was compiled before the changes were put forward earlier this week - Mr Straw said the criteria for holding inquests in private had since been "significantly tightened".
Its chairman, the Labour MP Andrew Dismore, said: "Whilst the government has attempted to modify the proposals, no proper justification has ever been put forward and the secret inquest plan should just be scrapped.
"Any investigation into the way a loved one has died - especially in circumstances involving the state - must remain transparent and accountable, and the bereaved must retain the right to be involved in the proceedings."
The report pointed out that allowing an inquest to be held in private to protect diplomatic relations between countries could apply in cases of British soldiers killed in "friendly fire" incidents involving US personnel.
Military families have raised concerns about the plans and Shami Chakrabarti, director of human rights organisation Liberty, said it was "perfectly possible to protect sensitive material within an open jury system".
Amnesty International director Tim Hancock added: "Coroners can already decide, independently, to exclude the public from part of an inquest on grounds of national security".
"Letting the government make this decision will lead to accusations of cover-ups," he said.
Earlier this week Mr Straw said he intended to "fundamentally recast" measures regarding inquests in light of concerns among MPs.
He hoped coroners would still be able to give a broad outline of sensitive information to juries - so secret inquests would be "few and far between".
But he said there would be circumstances when a judge might decide the only way sensitive information could be "the subject of a proper judgment by the court, but also protected itself, will be for the judge to sit alone without a jury".
Liberal Democrat justice spokesman David Howarth said: "The fundamental problem with the government's proposals is that they allow inquests to be held without juries when a person has died at the hands of the state.
"This is simply wrong. The latest amendments go some way to allaying some objections but not that most fundamental one."