By Grant Ferrett BBC correspondent, recently in Zimbabwe

As aid agencies warn of the growing threat of starvation to millions of people in Zimbabwe, the BBC has uncovered evidence of political interference in the distribution of food by President Robert Mugabe's supporters.

Some families have resorted to eating wild leaves and fruit

Food aid has been blocked in some areas which voted against Mr Mugabe during elections in March.

Opposition supporters have also been prevented from buying commercial stocks of grain.

Last week, President Robert Mugabe said claims of political interference came from the opposition and were without substance.

The signs of food shortages and hunger are everywhere in Zimbabwe.

Groups of villagers wait for days at a time by the roadside for deliveries of scarce supplies of the staple food, maize.

In towns and cities, long queues form outside supermarkets when stocks arrive.

The shelves are bare of basics such as sugar and salt.

Foraging for food

Many people in urban areas are reduced to eating a single meal a day, but the hardship is most severe in villages.

[The children] don't concentrate and they are weak and they are always complaining of hunger... you can't learn on an empty stomach

Schoolteacher

"From six o'clock in the morning, up to late, no food," 80-year-old Anderson Mudimba told me.

"We don't know... who will help us to have food, because we've waited and waited, and no assistance at all is coming."

I met families surviving on leaves and wild fruit.

They should be receiving food aid, but say it has been blocked by government supporters who accuse them of voting for the opposition.

Numerous interviewees said they had been refused permission to buy food from government grain depots unless they produced a ruling party membership card.

Prosper, a former teacher from Buhera, who fled his home after being denied food because he was an MDC supporter told me:

"There was a war veteran who said, 'You MDC, you are not going to buy this food, to buy maize, go back to the back of the line.'"

"So when you reach the number, they start taking you back again."

Children dying

Hospital officials in the north-western town of Binga, where the aid effort is being held up by Mr Mugabe's followers, say nearly 30 children have died in recent weeks from malnutrition-related illness.

We have people who are starving in the communities, but the war veterans are not allowing us to distribute the food

Local charity spokesman

Others have died after eating poisonous roots.

Many children no longer attend school as their days are spent searching for food, those that do make the journey to the schoolhouse are severely weakened by hunger.

"They don't concentrate and they are weak and they are always complaining of hunger," a schoolteacher told me.

"They also talk about ... [how] there's nothing at home to eat."

She said the children's hunger means it is impossible for them to concentrate on their work.

"[After school break] They don't faint as such, but they will be sleepy, which really shows signs of hunger," she said.

"You can't learn on an empty stomach, you can only concentrate if you have had enough."

Government control

Aid agencies complain that the government is trying to control the relief effort at every level.

	:	Southern Africa famine
In pictures: Southern Africa famine

One local charity official, who did not want to give me his name for fear of retribution, said that the situation was especially terrible as it need not occur.

"For us it is sad, especially when we have food around and we have people who are starving in the communities," he said.

"But the war veterans are not allowing us to distribute the food there."

Zimbabwe has suffered two poor rainy seasons in a row, but the crisis unfolding now is as much man-made as natural.

Grant Ferrett was reporting for BBC Radio 4's File on Four programme.

Its investigation into the crisis facing Zimbabwe will be broadcast on 30 July at 1900 GMT (2000 BST) and will be streamed on the website at http://www.bbc.co.uk/radio4/news/fileon4.shtml