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Labour's Jeremy Corbyn is hosting a meeting with other opposition party leaders later to discuss their approach to the timing of a general election.
MPs get another chance on Monday to vote for Boris Johnson's call for a snap election, after rejecting it earlier this week.
But Labour's Emily Thornberry called the PM "slippery", adding that he could use a poll as a "distraction".
Mr Johnson has said he would "rather be dead in a ditch" than delay Brexit.
It is the end of a tumultuous week for Boris Johnson, who has suffered a series of defeats over Brexit in Parliament, expelled 21 of his own MPs for rebelling and seen his younger brother resign from government.
Jo Johnson quit as a minister on Thursday and announced he was standing down as an MP, saying he had been "torn between family loyalty and the national interest".
The prime minister's decision to prorogue - suspend - Parliament next week ahead of a Queen's Speech on 14 October is also being challenged in the courts in Scotland and Northern Ireland.
But campaigner Gina Miller has lost a judicial review she brought to London's High Court over the prorogation. However, permission has been granted for the case to be heard in the UK Supreme Court on 17 September.
Also on Friday, Mr Johnson is in Scotland to announce additional funding for farmers. He will later visit the Queen's Balmoral estate.
Meanwhile, a bill designed to prevent a no-deal Brexit is expected to finish its progress through the House of Lords.
Shadow foreign secretary Ms Thornberry told BBC Radio 4's Today programme that the possibility of having an election was "extremely attractive" but the "immediate crisis" of preventing a no-deal Brexit had to be dealt with first.
"If we vote to have a general election then, no matter what it is that Boris Johnson promises, it is up to him to advise the Queen when the general election should be," she said.
Ms Thornberry said Mr Johnson was "as slippery as can be" and could not be trusted over the timing of polling day.
She said that because Mr Johnson had said he would "die in a ditch" rather than delay Brexit beyond 31 October, "our first priority has to be that we must stop no-deal and we must make sure that is going to happen".
"We have a prime minister who is so unlike any other prime minister that we have had. In the past, if you passed a law you could be pretty sure the prime minister will abide by that law," she said.
"But we heard from the prime minister's own mouth that he will die in a ditch - obviously I hope he doesn't, but I actually hope he would obey the law."
In a speech on Thursday, Mr Johnson reiterated his call for an election, which he wants to take place on 15 October - days before the UK is due to leave the EU on 31 October.
He argued it was "the only way to get this thing [Brexit] moving".
"We either go forward with our plan to get a deal, take the country out on 31 October which we can or else somebody else should be allowed to see if they can keep us in beyond 31 October," Mr Johnson said.
But during his visit to the West Yorkshire town of Morley, a member of the public told Mr Johnson he should be negotiating the UK's exit from the European Union, not travelling around the UK.
In the Lords, peers will continue debating a Labour-backed bill designed to block a no-deal exit on 31 October, after MPs passed the legislation on Wednesday.
All stages of the bill are due to be completed in the Lords by 17:00 BST, at which point the bill can go back to the House of Commons before being presented for royal assent.
Meanwhile, a judgement is expected on a challenge to the suspension of Parliament brought by businesswoman Gina Miller and former Conservative prime minister John Major in England's High Court.
In Belfast, the Royal Courts of Justice will hear a separate case arguing that the suspension is unconstitutional. Meanwhile, a group of politicians in Scotland are attempting to overturn a court ruling made on Wednesday that the plan to shut down Parliament is legal.