The government will “test to the limit” a new law designed to force it to seek an extension to the Brexit deadline if a deal is not reached by 19 October.
Foreign Secretary Dominic Raab said the government would abide by the law but would “look very carefully” at its “interpretation” of the legislation.
He said Britain remained committed to getting a deal with the EU.
It comes after Prime Minister Boris Johnson was warned he could face legal action if he chose to flout the law.
Shadow chancellor John McDonnell said the country was in “an extremely serious constitutional position” and that “no-one can trust” what might happen with Mr Johnson as PM.
“We’ve got to prevent Boris Johnson from forcing through a no-deal because of the damage it could do for our country,” he told the BBC’s Andrew Marr programme
Mr McDonnell said he believed the prime minister might wait until no-deal was the only option, and an election would not solve the problem.
He said: “I think we’re in an extremely serious constitutional position.
“We don’t believe that we can pin him down and I don’t trust him an inch. I don’t think anyone does.
What is the new law?
The bill, which is set to receive royal assent on Monday, was presented by the Labour MP Hilary Benn and backed by opposition parties and the recently expelled Tory MPs.
It gives Mr Johnson until 19 October to either pass a deal in Parliament or get MPs to approve a no-deal Brexit.
After this deadline, he would have to write to the EU asking for an extension to the UK’s departure date from 31 October to 31 January 2020.
The bill outlines the wording of the letter that the prime minister would have to write to the president of the European Council..
If the EU proposes a different date, the PM must accept it within two days.
But during this two-day period, MPs – not the government – would be able to reject the EU’s date.
Ministers will also be compelled to give the House of Commons Brexit progress updates over the following months.