Brexit has introduced a "poison" into UK politics that could lead to Scottish independence, according to former Prime Minister John Major.

In a scathing attack on hardline Conservative Brexiteers, Sir John branded members of the Jacob Rees-Mogg-led European Research Group (ERG) as "zealots" who are determining UK Government policy.

Meanwhile, he claimed, the Labour party is being held ransom by hard-left activists at the expense of more moderate views.

He said these extreme viewpoints have led to an increase in hate crimes and could lead to the break-up of the UK.

Speaking on Tuesday at the University of Glasgow's John Smith Centre, the ex-Tory leader will say: "Currently, both the Conservative and Labour parties are being manipulated by fringe opinion.

"The rationale for extremists joining mainstream parties is logical: from within them, they can influence policy; from without, they very rarely can.

"At the moment, there are people who - for now - may have their boots within the Conservative or Labour parties - but not in their minds, nor their hearts.

"The Conservative Party membership appears to be 'hollowing out' traditional Conservatives, while former Ukip members strengthen the anti-European right of the party."

Sir John will attack the "more extreme" members of the ERG for having no affinity to tolerant conservatism.

The former PM will say: "In Parliament, the European Research Group has become a party within a party, with its own whips, its own funding and its own priorities.

"Some of its more extreme members have little or no affinity to moderate, pragmatic and tolerant Conservatism.

"The ERG does not represent a majority view but - with a minority Government, as now - can determine policy simply by being intransigent: which is precisely what it is doing.

"Some - who can fairly be called zealots - seem incapable of looking beyond the one issue of Europe. It's not just that it dominates their thinking - it seems to obsess them."

Sir John will say that Labour has a "different dilemma", as control of policy is now at "the mercy of a passionate, active, far-Left base rather than the centre-Left."

The ex-PM will say: "Yesterday, seven moderate MPs left the Labour Party. I admire their courage and their conviction. But I hope they have not cut themselves adrift forever.

"Labour needs moderate MPs, and the country needs a moderate Labour Party.

"Complacent voices dismiss the chances of fringe opinion gaining control of the political agenda. Britain is too pragmatic, they say. Too stable. Our political system is too mature. I hope they are right."

The ex-Tory leader will say that various factors, including Brexit - which "unleashed a poison into our political system", have led to voters losing trust in politicians.

Sir John will also take a swipe at Theresa May's Brexit strategy, saying: "And there was nothing rational about triggering Article 50 before knowing what we wished to negotiate, or were prepared to concede to obtain it."

It is no coincidence hate crimes have increased since the EU referendum, the former PM will say.

"Foreigners have been made to feel so unwelcome, that many have already chosen to return home. The hostility directed at them will not be forgotten, and may grow into a distaste for our country that - one day - will rebound on us.

"The referendum was peppered, day after day, with anti-immigrant stories. At its conclusion, hostility to migrants had increased, and was made to seem acceptable. It is no coincidence that hate crimes have grown in number."

Sir John will also express concern that Brexit could break up the UK.

"As a Briton, I find it painful to come to terms with the possibility that, within my lifetime, I could - I repeat could - see the break up of the UK. I hope I am wrong, but the agonies of Brexit have the capacity to do just that."

Calling for a return to the centre ground of politics, Sir John will say: "When I refer to 'the centre', I don't mean some amorphous new party of 'moderates' and 'centrists'.

"Even if such a party were elected, what would unfold when it fell out of favour?

"With mainstream opinion sidelined, the country's only choice would be between the extremes of Left or Right. That would be an awful outcome. Our electorate needs a choice between parties that are demonstrably rational, realistic - and sane.

"So, when I speak of 'the centre', I mean that our three main national parties - Conservative, Labour and Liberal Democrat - must each retain a mainstream majority of their own."