The home secretary has been told the Parliamentary oversight proposed as part of efforts to extend detention limits to 42 days is a "charade".
Ms Smith said the proposals were the best chance for a consensus
Labour backbencher David Winnick said it was a "cosmetic exercise" as, by the time MPs voted, a suspect might already have been incarcerated for 42 days.
But Jacqui Smith told a committe of MPs the "spectre" of future scrutiny would hang over any detention decisions.
Ms Smith has also briefed Labour MPs on the government's plans.
Under the proposals, unveiled last week, the home secretary would be able to allow a terror suspect to be held without charge for up to 42-days - if a joint report by a chief constable and the Director of Public Prosecutions supported it.
But it would need to be approved by Parliament within 30 days.
Ms Smith confirmed, during a home affairs committee session, that this meant it would be possible that someone could be held for 42 days before Parliament even voted on the matter.
But, she said, having to answer to Parliament would "act as a safeguard" as it would put pressure on the home secretary making the decision, who would know they had to be "accountable to Parliament".
And she said, for any detention longer than 28 days, there would have to be an application to a judge, an adversarial hearing and a judicial agreement.
'Battles to come'
Mr Clappison said he agreed "there may be a spectre there", but he added "whether that amounts to scrutiny and a safeguard is a different matter".
He said, from the perspective of the person being held, to describe it as a safeguard was "misleading" and described it as a "flawed process" and a "charade". Mr Winnick remarked there were "battles to come".
Ministers face opposition from Tory, Lib Dem and some Labour MPs in any attempt to extend the detention limit.
In 2005, Tony Blair suffered his first Commons defeat as prime minister after seeking to extend the detention without charge to 90 days.
Mr Winnick suggested that the 42-day figure had been chosen as it was "the most likely" to gather sufficient Labour backbench support.
Ms Smith disagreed, adding: "It's being done, not to buy political support but to provide the police and those that we task with protecting us from terrorism with the tools that they need to do the job."
Asked whether she thought she would get enough Labour support for a maximum 42-day limit, she said she had left "number crunching" behind when she left her previous job as chief whip.
But she said the "reasonableness of the proposals" meant they were "the best chance of building the consensus necessary".
Ms Smith is due to brief Labour MPs - including those who voted against the 90 day extension - on the plans on Tuesday evening.
And the Home Affairs Committee will publish its own report on Thursday. Committee chairman Keith Vaz said Ms Smith had delivered an "impressive" performance and was clearly convinced that she had found a solution.
But he pointed out that of 71 responses to a Home Office consultation, only six had been "unequivocally" in favour of an extension on the current 28-day limit.
And he said the "only voices" in favour of an extension were the police and the government, as the director of public prosecutions, Sir Ken Macdonald, had indicated that he was happy with the 28 day limit.
It has been argued that the increasing complexity of terrorist plots mean police will need to hold suspects for longer in future.
Liberal Democrat home affairs spokesman Nick Clegg said only a "tiny minority" of people responding to the government's own consultation supported extending the pre-charge detention period.
"The more we hear of the government's arguments, the less convincing they become," he said.
"There is simply no case for reopening the debate on 28 days."