Boris Johnson could have to legislate for Northern Ireland to be placed under direct rule from Westminster if the Stormont Assembly is not up and running at the time of Brexit.
Dominic Raab, effectively Mr Johnson's deputy prime minister, said legislation would need to be considered if there was a no-deal Brexit so there would not be a "vacuum" in Northern Ireland.
Downing Street said the necessary "administrative, regulatory and legislative" arrangements would be in place if there was not a devolved government on October 31.
The Institute for Government said legislation would be needed to introduce direct rule from October 31 "given the scale and the speed of the interventions likely to be necessary".
The think tank said: "This will be extremely contentious, but without it Northern Ireland will be left even more exposed to the economic shocks of a no-deal Brexit than it is currently. That would itself raise the risk of all political backlash."
The Government insisted it was focused on ensuring the return of the Stormont government.
Asked if direct rule was more than likely to be the case in the event of a no-deal Brexit, Mr Raab said: "We'll make sure that the arrangements are in place so that there is no vacuum, that there is the efficient conduct of government.
"The number one priority is to see the parties in Northern Ireland revive the Executive and the Assembly so that actually they can take responsibility and control."
The Foreign Secretary and First Secretary of State said: "There will of course need to be legislation considered, across the no-deal scenario.
"We'll make sure that all the arrangements, whether they are regulatory or administrative, are in place so that we don't have a vacuum."
Asked whether direct rule could be done without legislation, Mr Raab told BBC Radio 4's Today programme that the question is "the extent to which it can be done".
He added: "What I'm saying is we will make sure the arrangements are in place to make sure all the risks across all quarters of the United Kingdom, but particularly in relation to Northern Ireland, can be addressed and managed in as sensible and as smooth a way as possible.
"And we are mindful, of course, of the sensitivities in Northern Ireland. The number one thing we have made clear is there will be no return to a hard border and no extra infrastructure."
A Number 10 spokeswoman said the focus was on restoring the power-sharing Executive, but "we will make sure that we have the administrative, regulatory and legislative arrangements in place should that not be the case".
"But our priority is to make that happen. The Prime Minister will be travelling to Northern Ireland as a matter of priority to talk to the parties and to do everything possible to ensure that the Executive and the Assembly is back up and running."
