Cameron returned as Iraq's new prime minister-designate won endorsements from uneasy mutual allies the United States and Iran and called on political leaders to end crippling feuds that have let jihadists seize a third of the country.
Cameron had been due to return from his summer holiday in Portugal on Thursday, but his office issued a statement on Wednesday saying he was already back in Britain and would chair a meeting of the government's emergency response committee.
Several lawmakers have called on Cameron to recall parliament from its summer recess to discuss Iraq, while at least two former senior military figures have said Britain should follow the U.S. lead and intervene militarily against Islamist fighters there on humanitarian grounds.
Britain is already conducting aid drops from C-130 cargo planes to try to help refugees trapped in northern Iraq and has sent Tornado jets and Chinook helicopters to help the humanitarian effort.
London has also agreed to transport critical military re-supplies being provided by other contributing nations to Kurdish forces to help them protect refugees in Iraqi Kurdistan from militants of the Islamic State (IS).