Former prime minister Gordon Brown has warned the future of the United Kingdom is at risk from the rise of "narrow nationalism" that makes people "hate their neighbours".
In a speech to the Fabian Society and Hope Not Hate, Mr Brown said the union has never been more at risk in its 300-year history, and criticised people like Nigel Farage and Boris Johnson for "hijacking" patriotism.
He said he feared the Conservatives would play fast and loose with the union, and it was "not safe" in the hands of Mr Johnson".
"I believe the Union is today more at risk than at any time in 300 years - and more in danger than when we had to fight for it in 2014 during a bitter Scottish referendum." he told an audience at Westminster Cathedral Hall.
"In jeopardy are both the unity and integrity of the United Kingdom and the shared values - tolerance, respect for diversity, being outward looking - that underpin what, for all its ups and downs, has been the most successful example of multinational co-operation anywhere in the world.
"In our long history the overwhelming majority have prided ourselves in being patriots who love our country - not bitter nationalists who hate our neighbours, demonise foreigners, immigrants or other minorities and blame external forces for everything that goes wrong."
Mr Brown called for consideration not solely of "the kind of Brexit we want" but of "the kind of Britain we aspire to become" and he criticised the front runner in the Tory leadership campaign, Mr Johnson.
He said: "You can love your country without being made to feel you ever have to hate your neighbour.
"You can embrace a broad patriotism without subscribing to a narrow nationalism.
"I want to argue specifically against the hijacking of patriotism by Nigel Farage and Boris Johnson - a political deception that has tried to present an act of economic self-harm - a no deal Brexit on October 31st - as a patriotic act."
He accused Mr Farage, now leader of the Brexit Party, and Mr Johnson of using nationalism to demonise immigrants, Europeans and minorities.
He added: "I want to argue we need an informed region-by-region debate outside the Westminster bubble through the setting up of citizens' assemblies on the problems raised by Brexit including immigration and sovereignty but many of which - the state of our manufacturing, the condition of our towns, and rising poverty and inequality - cannot be solved by Brexit.
"And I want to argue for a progressive defence of the union showing that we - all four nations - are best placed to succeed in a harshly competitive global economy - when we find ways to cooperate within one set of islands - rather than engage in economic wars."
This is not the first time the former chancellor, who served under Tony Blair's premiership for a decade, has been vocal about the union. He backed the Better Together campaign during the Scottish Independence referendum in 2014, and has been critical of the "extreme" independence of the SNP.
In a column for the Daily Mail, he wrote: "For just as all political attention has been fixated on Britain moving from a soft Brexit to a hard Brexit, the Scottish nationalists have moved - almost unnoticed - from demanding a soft form of separation to demanding a hard, far more extreme, type."
Mr Brown criticised Mr Johnson's record on Scotland, saying he repeatedly focused on the nation being "grossly over-represented" in parliament, and telling Scotland to "hop it".
In his column, he added: "So his knee-jerk response to counter a divisive us-versus-them Scottish nationalism is to embrace an equally divisive us-versus-them English nationalism."
Mr Brown's speech comes after the de facto deputy prime minister David Lidington announced his support for Jeremy Hunt, running against Mr Johnson, and said English nationalism was posing a new threat to the union.
He said: "I am a very strong unionist, but I think we need to be alive to the fact that there is a combination of nationalist feeling on the one hand, and indifference towards or ignorance of the value of the Union on the other, that puts that achievement at risk."
A YouGov poll found 63% of UK Conservative Party members would rather see Scotland leave the UK than dump Brexit.