Foreign Secretary Liz Truss said the hit on the Black Sea port of Odessa was "completely unwarranted", adding that "not a word" Russian President Vladimir Putin says can be trusted.
"It is absolutely appalling that only a day after striking this deal, Vladimir Putin has launched a completely unwarranted attack on Odessa," Ms Truss said at a campaign event on Saturday.
"It shows that not a word he says can be trusted. And we need to urgently work with our international partners to find a better way of getting the grain out of Ukraine that doesn't involve Russia and their broken promises," she added.
Russian missiles hit Ukraine's Black Sea port of Odessa on Friday, just hours after Moscow and Kyiv signed deals to allow grain exports to resume from the southern city.
Two Russian missiles hit the port's infrastructure and Ukrainian air defences brought down two others, the Ukrainian military said.
"It took less than 24 hours for Russia to launch a missile attack on Odessa's port, breaking its promises and undermining its commitments before the UN and Turkey under the Istanbul agreement," Foreign Ministry spokesman Oleg Nikolenko said.
Mr Nikolenko described the missile strike on the 150th day of Russia's war in Ukraine as Russian President Vladimir Putin's "spit in the face of UN Secretary-General Antonio Guterres and Turkish President Recep Tayyip Erdogan, who made great efforts to reach agreement".
The deals were intended to clear the way for the shipment of millions of tons of Ukrainian grain and some Russian exports of grain and fertiliser held up by the war.
WATCH: Armed Forces Minister James Heappey discusses whether UK minehunters should be sent to the Black Sea to help.
Russian Defence Ministry spokesman Igor Konashenkov said at a daily briefing on Sunday that the air strike had targeted only Ukrainian military facilities.
"In the seaport in the city of Odessa, on the territory of a shipyard, sea-based, high-precision, long-range missiles destroyed a docked Ukrainian warship and a warehouse with Harpoon anti-ship missiles supplied by the US to the Kyiv regime," he said.
Ukrainian President Volodymyr Zelensky said in his nightly video address on Friday that the agreements offer "a chance to prevent a global catastrophe – a famine that could lead to political chaos in many countries of the world, in particular in the countries that help us".
Ukraine is one of the world's largest exporters of wheat, corn and sunflower oil, but Russia's invasion of the country and naval blockade of its ports halted shipments.
Conservative MP and Defence Select Committee chair Tobias Ellwood left Odessa earlier in the week.
Commenting on the events, he said he was "baffled by how the international community was bluffed" by Russia.