Video report by ITV News UK Editor Paul Brand
While another referendum seems more likely than ever, Mr Brown is leading a campaign to stop the split.
He, however, has condemned Mr Johnson's efforts to keep Scotland part of the UK, calling it "muscular unionism".
Gordon Brown on how Boris Johnson has handled calls for a referendum
"It's putting up more flags, it's badging bridges and roads with a UK flag and it's bypassing the Scottish government and the Scottish institutions," Mr Brown said.
"It won’t work".He also warned the prime minister against forging competition between the rest of the UK and Scotland.
“What people want is Scotland and Britain to work together, not a competition between the two,” he said.
Listen to the ITV News politics podcast, Calling Peston
Mr Brown was staunchly against Scottish independence in the run up to the first referendum in 2014. However, he says the pandemic has proven the United Kingdom doesn't serve its nations and regions as well as it should do.
A poll made for ITV News in March revealed some of the highest levels of support for Welsh independence ever recorded, while post-Brexit anger has been bubbling in Northern Ireland during the outbreak.
To address such dissatisfaction, Mr Brown said Mr Johnson must "get on with reforming the United Kingdom" so it appeals to its constituent parts.
"By that I include Wales, the metro mayors in all the regions," he continued.
"They're all complaining they don't have enough power to do the things they want".
Mr Brown added that alongside the union, the PM's approach to the SNP needs a rethink.
"Now that's completely at odds with the opinions of the vast majority of Scottish people.
“He thinks he’s attacking the SNP, he’s actually attacking mainstream Scottish public opinion."
Mr Brown ardently believes the PM's job is to try and unite the country - "he cannot be at war with part of it”.