Preparing for the possibility of a no-deal Brexit should be "the top priority" for civil servants, Boris Johnson has told them in a letter.
The PM said he would prefer to get a deal with the EU, but he said he recognised this "may not happen".
Earlier Jeremy Corbyn had urged the UK's top civil servant to intervene to prevent a no-deal Brexit happening during a general election campaign.
It comes amid speculation MPs could back a no-confidence motion in the PM.
In his letter to civil servants, Mr Johnson said the UK must be prepared to leave the EU by the latest Brexit deadline of 31 October "whatever the circumstances".
"That is why preparing urgently and rapidly for the possibility of an exit without a deal will be my top priority, and it will be the top priority for the civil service too."
It is understood that government special advisers also received an email last night from the PM's senior adviser Eddie Lister instructing them not to take annual leave until after 31 October.
Mr Johnson's message to civil servants follows a similar letter sent by Chancellor Sajid Javid earlier this month to HM Revenue and Customs.
Mr Javid also ordered the tax authority to make preparing for no-deal its "absolute top priority", including helping the public to prepare for the possibility.
He said this should include making sure IT systems are ready, helping businesses with a helpline, and contacting traders directly.
In his letter to Cabinet Secretary Sir Mark Sedwill, Mr Corbyn said it would be an "anti-democratic abuse of power" if the PM allowed a no-deal to occur by default during a general election campaign, if the government was defeated in a vote of no confidence.
Shadow chancellor John McDonnell has said it is "almost inevitable" that Labour would push for such a vote when the Commons returns from its summer recess on 3 September.
It is thought MPs opposed to no-deal could back the vote in a bid to prevent the UK leaving the EU without an agreement - leading to a general election being called.
Election rules say Parliament should be dissolved 25 working days before polling day - so some people are concerned Mr Johnson could allow a no-deal Brexit to happen while MPs are not sitting.
According to the Sunday Telegraph, Mr Johnson's senior adviser at No 10, Dominic Cummings, has told MPs even losing such a vote could not stop the PM taking the UK out of the EU on 31 October.
He reportedly said Mr Johnson could call an election for after the deadline, with Brexit taking place in the meantime.
Theresa May's Brexit deal was rejected three times by MPs and, as things stand, the UK will leave the EU on 31 October whether it has agreed a new one or not.
Mr Johnson has urged the EU to make changes to the deal, but has said the UK must leave by this deadline with or without an agreement.
Many of those who voted against the deal had concerns over the backstop, which if implemented, would see Northern Ireland staying aligned to some rules of the EU single market.
It would also see the UK stay in a single customs territory with the EU, and align with current and future EU rules on competition and state aid.
These arrangements would apply unless and until both the EU and UK agreed they were no longer necessary.
On Thursday Mr Johnson again urged the EU to compromise on the Irish border backstop plan, designed to guarantee there will not be a hard Irish border after Brexit.
However, the EU has continued to insist that the withdrawal deal agreed by Mrs May last year, including the backstop, cannot be renegotiated.
Meanwhile, a transport minister has said he supports the government position of leaving the EU in all circumstances, following comments he made about a no-deal Brexit.
George Freeman told HuffPost UK's Commons People podcast it would be an "absolute disaster" for the UK in the long term if it only traded with the EU on WTO [World Trade Organization] terms, without its own free trade deal with the bloc.
But he later tweeted to say he "totally supports" the position that the UK should leave with no deal on 31 October, "if the EU is unwilling to negotiate".
In response, Downing Street told the BBC that Mr Freeman has the "full support of the prime minister".