Their decades-old rivalry is so intense that Burger King once mocked the McDonald's mascot with a horror advert warning "Never trust a clown".
But such is the crisis sweeping the fast food industry that the Whopper seller is now seeking to prop up its old foe - and has launched a new advert urging customers to help McDonald's stay afloat.
The move is part of a media campaign to highlight the plight of the hospitality industry as countries across Europe head into a second lockdown with millions of livelihoods at risk.
Burger King said: "We never thought we'd be asking you to do this, but restaurants employing thousands of staff really need your support at the moment.
"So, if you want to help, keep treating yourself to tasty meals through home delivery, takeaway or drive thru. Getting a Whopper is always best, but ordering a Big Mac is also not such a bad thing."
The crisis has taken a particular toll on the restaurant industries, with businesses forced to shut for the majority of the spring now suffering a fresh blow.
In September, it was reported that Burger King was preparing to close up to 25 of its UK restaurants as part of a restructuring process.
The company said that its show of goodwill was not exclusively directed at McDonald's, adding that it was encouraging customers to purchase food "from any of our sister food chains (fast or not so fast)".
Alasdair Murdoch, chief executive of Burger King UK, previously criticised the Government’s policy towards hospitality during the pandemic.
In March he said the chain would not be paying its quarterly rent bill, writing in an open letter to landlords: "This crisis affects the whole UK economy, which will take time to recover.
Mr Murdoch highlighted the industry's importance, pointing out that it is the third-largest private sector employer with 3.2m workers and is twice as large as financial services.