Boris Johnson must reinstate the 21 ousted Conservative MPs or risk the party being reduced to a “mean-minded sect”, John Major is expected to say in a speech.

Speaking at a CBI dinner in Glasgow on Thursday evening, the former prime minister will also take aim at Johnson’s advisers including Dominic Cummings, saying they could “poison the political atmosphere beyond repair”.

“The legitimate concerns of those who have been banished from the party … seem to be worth nothing – unless they become cyphers, parroting the views of a prime minister influenced by a political anarchist, who cares not a fig for the future of the party I have served,” Major is scheduled to say.

“We have seen over-mighty advisers before. It is a familiar script. It always ends badly. I offer the prime minister some friendly advice: get rid of these advisers before they poison the political atmosphere beyond repair. And do it quickly.”

Prominent MPs including former chancellors Philip Hammond and Ken Clarke, Winston Churchill’s grandson Nicholas Soames and ex-Tory leadership contender Rory Stewart were among those who had the whip withdrawn on Tuesday after they backed moves to stop a no-deal Brexit.

But Major will encourage the prime minister to invite those he effectively sacked back into the party. “Reinstate those members of parliament you have expelled because, without them – and others like them – we will cease to be a broad-based national party, and be seen as a mean-minded sect,” he will say.

In recent statements Major, a committed remainer, supported legal campaigner Gina Miller in her high court challenge earlier on Thursday to thwart the suspension of parliament.

In a previous attack on Johnson’s decision to prorogue parliament, Major described it as “utterly, utterly and completely the wrong way to proceed”.

While he will renew his warnings of the “pernicious effects” Brexit could have on the future of the nation, Major will also use the speech to warn of the possible consequences if Scotland were to seek independence from the UK.

“The English nationalists affect not to care about separation. They care more about leaving Europe. But the collapse of unionism in England, and ambition for independence in Scotland, could lead to a calamitous outcome for us both,” he will say.