David Cameron has taken the rare step of inviting acting Labour leader Harriet Harman to attend a meeting of the National Security Council (NSC) to consider the threat posed by Islamic State (IS) jihadis.
The meeting, expected to take place on Tuesday, comes as the Government attempts to build a consensus behind extending the RAF's air campaign to include strikes against IS targets in Syria.
The move, the first time a Labour leader has been invited to attend the top-level meeting since 2013, also follows the Government's call for Britons to leave Tunisia in response to the continued terror threat after the Sousse beach massacre.
Ministers have faced criticism over the decision to warn against all but essential travel to Tunisia, with protests from the North African country claiming the UK was playing into terrorists' hands and complaints from tourists forced to cut short their holidays.
Downing Street has insisted that "substantial" work was needed to improve security for tourists since gunman Seifeddine Rezgui murdered 38 holidaymakers, including 30 UK nationals, in an outrage for which IS - also known as Isil - has claimed responsibility.
As well as Ms Harman, shadow defence secretary Vernon Coaker has been invited to the NSC, with Downing Street sources indicating the Prime Minister believes it would be "helpful" for the Labour figures to attend "given recent events".
A Number 10 spokesman said: "The PM has invited Harriet Harman to attend next week's National Security Council meeting in Number 10, which will be discussing the threat posed by Isil.
"The PM thought it was important to ensure the Leader of the Opposition was fully briefed on the current situation. The shadow defence secretary, Vernon Coaker, will also attend."
The RAF is already carrying out strikes against IS targets in Iraq, but unlike the US is not conducting operations in Syria.
Mr Cameron believes the group needs to be confronted in its Syrian stronghold but is reluctant to ask MPs to vote on extending British military action into the country unless he can be assured of Labour's support.
With senior Conservative MPs continuing to their voice their opposition, the support of Labour is crucial if ministers are to be sure of a Commons majority.
In a reversal of their previous position last September - when Ed Miliband made clear they would block strikes on Syria - Mr Coaker has indicated Labour would "carefully consider" any new proposals from the Government.
A Labour spokesman said: "We can confirm Harriet Harman and Vernon Coaker will attend the NSC meeting on the invitation of the PM."
The NSC is the Government's main forum for collective discussion of the threats posed to the country's security and is attended by senior ministers, with military chiefs and the heads of the intelligence agencies also regular participants.