The SNP’s autumn conference is to open in Aberdeen.
The conference is being held as party leader Nicola Sturgeon faces mounting pressure from some members over her independence strategy.
Ms Sturgeon wants to hold a referendum next year – but the UK government has already ruled out granting the consent she says is needed to make it legal.
Some activists are growing impatient and have called on Ms Sturgeon to set out a so-called Plan B.
Speaking on the first day of the conference, the party’s Westminster leader Ian Blackford is expected to say the SNP will retain its majority of Scottish seats at Westminster to reaffirm its mandate to hold a second referendum “if that’s what it takes for any UK government to listen”.
He will tell delegates the “only option” to address the Brexit “crisis” is to call a general election.
He will say: “The excuses for keeping Boris Johnson in office are running dry, and the patience of people in Scotland is running out. We must take the power out of Boris Johnson’s hands – and put it back in the hands of the people.”
‘Leader in waiting’
There have been calls for an unofficial independence referendum – similar to the disputed one in Catalonia in 2017 – to be held as soon as possible.
SNP members, including MP Angus MacNeil and former health secretary Alex Neil, have argued that winning a majority of Scottish seats at Westminster should be enough for independence negotiations to begin without the need for a referendum – which was once the SNP’s official policy.
Mr MacNeil has suggested that high-profile SNP MP Joanna Cherry, who has led a series of successful legal challenges against the UK government over Brexit, is a “leader in waiting” for the party.
Ms Sturgeon has also been criticised by some within the Yes movement over her failure to attend any of the large All Under One Banner pro-independence rallies that have been held across Scotland, including one in Edinburgh last weekend.
The party’s spring conference in April saw party members inflict a minor defeat on the leadership over its proposals for an independent Scotland’s currency.
But while independence is likely to feature prominently in Ms Sturgeon’s speech, and in a roundtable session of Scotland’s future featuring deputy leader Keith Brown, MSP Kate Forbes and MP Mhairi Black, delegates will not be given the opportunity to debate indyref2 in the main conference hall.
This has angered a group of grassroots members including Mr MacNeil and Inverclyde councillor Chris McEleney, who have been among the most vocal over the need for a Plan B.
Mr McEleny is expected to attempt to have a debate on Plan B added to the conference agenda when the event opens.
Analysis by Philip Sim, BBC Scotland political reporter
Questions about the timing of indyref2 and what is being done to deliver it have swirled around SNP conferences since, well, indyref1.
But the party rarely proves itself to be as divided as its opponents hope. Twitter tirades aside, time and again the vast bulk of the membership prove themselves to be firmly behind Nicola Sturgeon.
Still, the leader is taking a tougher line with what rebels there are. Whereas previously she has politely asked for “patience” and urged members to keep the faith, her comments in the build-up to this conference have been stark: there is no shortcut to independence. Forget Plan B.
Or to put it another way, “keep playing with the heid”. Get the groundwork done, and we’ll have a referendum when we’re sure we can win it.
Ahead of the conference, Ms Sturgeon told BBC Scotland that a legal referendum similar to the one in 2014 remained the only way for the country to win independence, and warned critics of her approach that there was “no easy or shortcut route”.
Writing in the conference guide, she said support for independence was “on the rise across Scotland”, but urged party members to “reach outwards and engage those who are not yet persuaded”.
She also said the party would use any future general election to send a “crystal clear message that Scotland must have the right to choose independence, within the current Holyrood term of parliament, with the democratic mandate we already have”.
Ms Sturgeon said the conference was being held at a “time of great uncertainty”, adding: “Never in my lifetime has politics been so turbulent, and never in that time has the threat to Scotland’s wellbeing and prosperity been so profound.
“A Tory government led by a cabal of hardline extremists intent on pushing through Brexit, no matter the harm and damage it will cause, has led us to the brink of disaster.
“Scotland said no to Brexit and we meant it.”
What else is on the conference agenda?
The conference will open at 14:00 on Sunday with a welcome address by the party’s Westminster leader, Ian Blackford, and close with Ms Sturgeon’s keynote speech on Tuesday afternoon.
Among the topics being discussed in the main hall will be Brexit, climate justice, drugs policy, state pensions in an independent Scotland and the importance of the Scots language.
There will also be speeches from senior party figures including Deputy First Minister John Swinney and Finance Secretary Derek Mackay.