Gordon Brown will pledge to “fight, fight and fight again” to keep the United Kingdom intact, warning today that the union is at serious risk due to Conservative Party attitudes and the rise of “narrow nationalism”.

Speaking in Westminster Cathedral Hall this morning, the former Labour leader and Prime Minister will share his fears that the “unity and integrity” of the UK could be lost under the premiership of Boris Johnson.

Brown is expected to point out that the Tories, at the urging of Nigel Farage, have falsely portrayed a no deal Brexit, an “act of economic self-harm”, as a “patriotic act”. He will outline his concern that the governing party will deploy an “English card” at the next election, which would contribute to the risk of the UK breaking up.

The ex-PM will become the latest high-profile political figure to advocate the idea of citizens’ assemblies, suggesting that representative groups of citizens could be formed on a regional basis to tackle the problems raised by Brexit.

Addressing the Fabian Society and Hope Not Hate, Brown will say: “Noticeable by its absence – even as we enter the third week of the contest as to who is to be our Prime Minister – is any serious debate on the existential question facing the United Kingdom: whether it can survive.

“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.

“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.

“I fear for the unravelling of a community of mutual interests, common purpose and shared ideals. For the national debate is now more than about the kind of Brexit we want: it is about the kind of Britain we aspire to become.

“A tolerant country must not now become an intolerant one. An outward-looking country must not now turn in on itself.

“A fair-minded and inclusive people must not allow the vicious manufacture of division and the targeting of ‘enemies of the people’.

“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.

“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.”