The Burmese Government was under increasing international pressure last night as aid agencies warned the death toll following the cyclone disaster could reach 1.5 million.

Oxfam said the stricken country faced a public health catastrophe unless clean water and sanitation was quickly provided.

The military junta has been criticised for not allowing emergency supplies and skilled aid workers into the secretive south-east Asian country quickly enough.

Foreign Secretary David Miliband blamed the "malign neglect" of the Burmese regime for turning the disaster into a "humanitarian catastrophe of genuinely epic proportions".

Rain storms were expected to lash the already deluged region last night, raising fears the situation could deteriorate quickly.

It is feared the sluggish response to the crisis could result in a death toll dwarfing the devastating Boxing Day tsunami which killed 250,000 people.

An estimated 100,000 have already perished in the disaster but Oxfam warned this figure could multiply 15-fold.

Aid is continuing to trickle into the country, with the Red Cross confirming a total of 10 flights containing emergency supplies will have landed in the capital Rangoon by today.

Charity and disaster response organisations are finally starting to reach the far flung stretches of the Irrawaddy Delta in Southern Burma, which bore the brunt of Cyclone Nargis which struck more than a week ago. Survivors are beginning to enter towns after fleeing their shattered villages as they desperately look for food and water.

International Development Secretary Douglas Alexander confirmed yesterday a British government assessment team had been allowed into the country. And in a further move suggesting the Burmese regime was becoming more open to foreign help, the Thai Foreign Minister Dr Noppadon Pattam announced he would meet his Burmese counterpart in Rangoon on Tuesday to discuss getting more aid into the country.

But agencies complained visa restrictions on specialist staff and long clearance times on aid flown into the country were impeding their efforts.

In a further setback yesterday, a Red Cross boat carrying emergency supplies in the Irrawaddy Delta sank after hitting a submerged obstacle under the waterline.

The UN estimates that two million people have been displaced with the majority left without shelter and clean water. Large parts of the country's infrastructure have been ruined, with bloated corpses and the rotting remains of cattle littering the countryside.

The huge storm surge left behind filthy pools of stagnant water, creating the perfect breeding conditions for mosquitos carrying malaria and dengue fever.

Sarah Ireland, Oxfam's regional director for South East Asia said: "With the likelihood of 100,000 or more killed in the cyclone there are all the factors for a public health catastrophe which could multiply that death toll by up to 15 times in the coming period.

"In the Boxing Day tsunami, 250,000 lost their lives in the first few hours but we did not see an outbreak of disease because the host governments and world mobilised a massive aid effort to prevent it happening."

Mr Miliband criticised Burma's response to the disaster, saying in a TV interview: "A natural disaster is turning into a humanitarian catastrophe of genuinely epic proportions in significant part because of the malign neglect of the regime."

Despite the crisis, the Burmese authorities pushed ahead with a referendum on Saturday. Mr Miliband labelled the decision to proceed with the vote as "bizarre".

The Disasters Emergency Committee an umbrella group for the organisations dealing with the crisis said that donations of around £5m had now been made for an appeal fund for the crisis.