Labour is urging the government to publish its assessments of Suella Braverman's security breach following her controversial reappointment as home secretary just six days after she was forced out.
New Prime Minister Rishi Sunak is under growing pressure over reinstating Ms Braverman as home secretary after a former party chair claimed she had committed "multiple breaches" of the ministerial code.
He has so far resisted demands to launch an inquiry after she shared a sensitive document with a Tory backbencher from a personal email without permission.
Both Labour and the Liberal Democrats have raised "national security" concerns over the breach, as well as called for a Cabinet Office probe.
Labour will try to push ministers to share risk assessments of this and other alleged leaks, as well as the information given to Mr Sunak before he reinstalled her at the Home Office, with a "humble address" motion in Parliament.
Ms Braverman has so far refused to appear before MPs to explain the circumstances surrounding what happened.
Shadow Home Secretary Yvette Cooper said: "Rishi Sunak and Suella Braverman cannot keep running away from these questions. It is far too serious for that, and raises serious doubts about the prime minister's judgment.
"People need to know they can trust the Home Secretary with highly sensitive information and our national security. Rishi Sunak's decision to reappoint Suella Braverman was deeply irresponsible.
"Labour will use every parliamentary mechanism open to force government to come clean over her reappointment, to get answers and to require detailed documents to be released to the Intelligence & Security Committee."
Ms Braverman resigned from her post just over a week ago after using her personal email address to forward sensitive government documents, breaking the rules that ministers have to abide by.
But the new prime minister put her back into the Home Office on Tuesday, and stood by the decision after being pressed on it in the Commons on Wednesday.
Labour leader Sir Keir Starmer has called for the home secretary to be sacked and accused Mr Sunak of brokering a "grubby deal trading security for support" in the Tory leadership contest, which he won after receiving Ms Braverman's backing.
But Mr Sunak has insisted the home secretary has "learned from her mistake" and that he does not regret his decision to reappoint her - despite some Conservative MPs adding their voices to the backlash.
Conservative MP Caroline Nokes said there were "big questions hanging over this whole issue" and backed opposition calls for an inquiry, while former Tory chairman Sir Jake Berry described the breach as "really serious".
Shadow foreign secretary David Lammy called for a full investigation into Ms Braverman's actions and for her to be sacked, telling Sky News: "The home secretary is the most serious job you could have in our state.
"This is a person who makes judgements about terrorism and counter-terrorism, who makes judgements about very, very serious offenders, whether they should be allowed out of prison, and for that reason, it's someone who, I'm afraid, judgement is critically important.
"I'm afraid this is a lapse of judgement that, quite rightly, she was sacked for. The question is, why was she brought back?"
Dubbed "Leaky Sue", Ms Braverman was reportedly previously investigated by government officials after the leaking of a story involving the security services.