The Prime Minister argued leaders need to now consider “plan B” as he pledged British expertise to help de-mine the Back Sea and provide further weapons.
Mr Johnson wants to offer insurance for commercial vessels to go free the 23 million tonnes of grain trapped by the Russian president’s blockade.
The UN is trying to alleviate a global food crisis by calling on Nato member Turkey and Russia to agree a safe corridor for exports.
Mr Johnson told reporters: “Now the problem is that Putin is going to use that as a pretext, as a way to try and get sanctions relaxed.
“And he is going to say I’ll let the 23 million tonnes of grain out if you’ll go easy on this, this and this.
“I don’t think that’s a runner.
“So we have to think about a plan B which would be to find ways of empowering the Ukrainians to control the sea lanes from the shore with various bits of kit.
“And there are two things in particular which the UK has expertise in.
“One is de-mining, remote de-mining, and the other is insurance of commercial vessels in contested waters and how to make it possible for people to take on that job.”
He said Britain is in talks with Ukrainian president Volodymyr Zelensky and the Turks to get an agreement.
“At some stage the world is going to have to move from Plan A which is the UN plan with Russian help, which I think is probably going to be a non-starter, to a plan B,” he added.
Mr Johnson insisted Turkish president Recep Tayyip Erdogan “definitely wants the grain out” when asked if he is too close to the Putin regime.
The Prime Minister said Ukraine is at a “critical point” in the war where it appears that the Russians are “running out of puff”.
“And you know, just mincing, mincing up the Ukrainians, let me choose a better word, can you strike mincing out, just keep grinding forward, that’s a better way of putting it, keep grinding forward, that’s a better way of putting it.
“That is the risk,” he added.
Mr Johnson was in Rwanda discussing the grain crisis with leaders at a Commonwealth summit and will travel on to G7 and Nato meetings in Europe.
He warned allies against becoming “exhausted with this thing”, arguing it would be a “disaster” for the world if Mr Putin wins ground.
“I’m going to make it today in the (leaders) retreat, but tomorrow in the G7. I think it’s just repeating that basic point that a victory for Putin is a disaster,” Mr Johnson said.
Foreign Secretary Liz Truss was also warning against getting “tired” support for Ukraine, as she warned against a peace deal that would be a “concession” to Moscow.
After holding talks with Turkey in Ankara, she told reporters in Kigali that “we’ve only got a month to do this”.
“My concern is that Russia is currently prevaricating,” she said.