Britain’s top civil servant has demanded ministers co-operate with his inquiry into the leaking of discussions at the National Security Council.
Sir Mark Sedwell has written to ministers on the council and their special advisers after the Daily Telegraph reported details of a meeting about Chinese telecoms firm Huawei.
It comes amid mounting pressure for a more robust response to the leak.
Several cabinet ministers have publicly denied they were involved.
Following Tuesday’s meeting of the council, the Daily Telegraph reported that it had agreed to allow Huawei limited access to help build Britain’s new 5G network, amid warnings about possible risks to national security.
The paper also reported that various ministers had raised concerns about the plan.
There has been a backlash against the leak, with Culture Secretary Jeremy Wright saying the government cannot rule out the possibility of a criminal investigation.
Cabinet Secretary Sir Mark is leading the internal inquiry.
Why does the leak matter?
The National Security Council (NSC) is made up of senior cabinet ministers and its weekly meetings are chaired by the prime minister, with other ministers, officials and senior figures from the armed forces and intelligence agencies invited when needed.
Its talks are supposed to be confidential.
It is a forum where secret intelligence can be shared by GCHQ, MI6 and MI5 with ministers, all of whom have signed the Official Secrets Act.
The BBC’s political correspondent Iain Watson said that while leaks from cabinet meetings are commonplace, information from the NSC had never previously been made public.
Former Defence Secretary Sir Michael Fallon told the BBC everyone involved in the council should be investigated to find out who the leaker is, while former Conservative chief whip Andrew Mitchell said the security services should be called in.
Amid speculation about who was behind the leak, several ministers took the unusual step of denying any involvement.
Home Secretary Sajid Javid said divulging sensitive information was “completely unacceptable”.
Defence Secretary Gavin Williamson and Foreign Secretary Jeremy Hunt denied the leak had come from them, with Mr Hunt calling it “utterly appalling”.
Sources close to International Trade Secretary Liam Fox and International Development Secretary Penny Mordaunt also denied divulging information.
According to the Daily Telegraph, Huawei would be allowed to help build the “non-core” parts of the UK’s 5G network, such as antennas.
There has been no formal confirmation of Huawei’s role in the 5G network and No 10 said a final decision would be made at the end of spring.
Huawei has denied there is any risk of spying or sabotage, or that it is controlled by the Chinese government.
When questioned about the issue, Prime Minister Theresa May said: “We don’t comment on leaks and on those matters.
“On the overall matter of security and our telecoms network, we are very clear that we give that high priority.
“We want to ensure we see greater resilience in our telecoms network and that we are able to provide high levels of cyber security, but we also see diversity of suppliers.”