The UN reacted furiously last night to Burma's military government confiscating food aid intended for more than a million victims of last week's cyclone. Two planeloads were impounded by the junta, prompting a temporary suspension in deliveries. UN flights were resumed last night, in the hope that negotiations would lead to a resolution.

Gordon Brown called the Burmese action "utterly unacceptable". He stopped short of joining France and the US in calling for aid deliveries without Burmese permission, although pressure within his government for such a move is growing.

Last night, the international development secretary, Douglas Alexander, said an emergency UN meeting today would demand urgent humanitarian access. A UN appeal launched yesterday raised almost half its target of $187m (£96m).

Britain's ambassador to the UN, Sir John Sawers, last night demanded that the Burmese government grant visas to aid workers, allow importation and distribution of food aid, and stop charging import duties on it. He also hinted Britain might back the distribution of aid without Burmese government permission.

"The United Nations will need to reflect on what steps it can take if a government prevents the international community from taking the steps needed to prevent an even greater disaster than the one we face now," Sawers said.

Britain's ambassador to Burma, Mark Canning, said authoritative estimates of the numbers of dead and missing ranged from 63,000 to 100,000, while up to 1.9 million people were vulnerable to water-borne disease, hunger and lack of drinkable water. "So you can do the maths and you will see how quickly this thing can get larger." Small amounts of aid were trickling through, he added, but not nearly enough.

Save the Children estimated that a tenth of the victims made homeless by Cyclone Nargis last Saturday had been reached by yesterday when aid deliveries were halted. Seven tonnes of high-energy biscuits flown in on a Thai commercial flight last Thursday were cleared by Burmese customs but the next two loads, 38 tonnes, enough to feed 95,000 people, were seized by the government.

"All of the food aid and equipment we managed to get in has been confiscated," Paul Risley, a spokesman for the UN's World Food Programme, said in Bangkok. The regime was also blocking visas for aid workers. The foreign ministry said it would accept cash and material aid but not international aid workers; the government would distribute food aid.

Britain is putting diplomatic pressure on Burma via China, India and Thailand. But if this fails in the next few days, it would look again at unilateral delivery. "If it comes to letting hundreds of thousands of people die, of course we're not going to do that," a British official said.

"There are people suffering," the prime minister told Sky TV. "It's utterly unacceptable that, when international aid is offered, the regime will try to prevent that getting in."

The row over food deliveries came after a standoff between the junta and the UN over visas for up to 40 disaster management experts.

UN secretary general Ban Ki-moon criticised the government's decision to press ahead today with a referendum strengthening the generals' grip on power.