Jolyon Maugham, a barrister and anti-Brexit campaigner, said there was "a flaw" in the so-called Benn Act, passed by parliament earlier this month, that could force MPs opposed to no-deal to take "counter-measures".
The loophole means that the prime minister would be able to lawfully take Britain out of the EU without a deal even if MPs had voted for an agreement, Mr Maugham said.
Under the terms of the Benn Act, Mr Johnson must ask the EU for a further delay to Brexit if MPs have not approved an exit deal by 19 October. If a deal is approved, the duty to request an extension would be overridden.
However, there is no legal obligation on the government to actually implement the terms of the deal, even if MPs have approved it. Mr Maugham suggested that Mr Johnson could therefore simply refuse to bring forward the Withdrawal Bill, which is needed to implement the deal in law.
In that situation, MPs would have approved a deal but it would not have been formally ratified in law, meaning the UK would leave the EU on 31 October without a deal in place.
The suggestion prompted speculation that Mr Johnson could tell Eurosceptic Tory MPs in the European Research Group (ERG) to vote for a deal while privately promising that he would not implement it. The prime minister hosted a dozen Tory MPs at Chequers on Friday.
Mr Maugham said the 2018 EU Withdrawal Act lays out "further obligations" that the government must meet before a deal can be fully ratified, including a new law implementing the agreement.
He wrote: "Summing up, if the Commons approves the withdrawal agreement but these further obligations are not satisfied before 31 October 2019, then two consequences follow. First, the Benn Act will not apply to require the PM to request an extension from the EU. And, second, we will leave with No Deal.
"So, imagine the PM says privately to the ERG ‘support my withdrawal agreement and I will deliver no-deal.’ In those circumstances, with the help of some Labour MPs, the Commons might approve even Theresa May’s withdrawal agreement
"The PM would thus have escaped the obligation in the Benn Act to request an extension and could deliver no-deal."
It follows reports that Dominic Cummings, Mr Johnson's top adviser, has told government aides that ministers could suspend parliament again in October if the Supreme Court, which will hear an appeal this week, rules that the original suspension was unlawful. No10 sources insisted the comment was "quite clearly a joke".
Mr Maugham speculated that Mr Johnson could suspended parliament again once MPs had voted in favour of a Brexit deal, thereby bypassing the Benn Act but denying his opponents the opportunity to pass further legislation blocking no-deal.
The barrister said he had discussed the loophole with MPs and urged them to take "counter-measures". This should include refusing to vote for any Brexit deal before 19 October, he said, adding: "Those who want the Withdrawal Agreement should refuse on the basis that, by voting for it, they may well be delivering No Deal."
He said Mr Johnson would then be forced to request a Brexit extension until 31 January, leaving plenty of time for the withdrawal agreement to be approved and fully ratified, if MPs agreed to it.
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