It was meant to be a show case for healthy living, with the first lady, Michelle Obama, personally putting hand to pitch fork in a crowd of school children to dig up the first White House vegetable garden in more than 50 years.

Instead, an embarrassed White House admitted today that the plot - whose lettuce, herbs and other produce have been consumed by the first family, visiting dignitaries, local school children and a women's homeless shelter - had tested positive for elevated levels of lead.

A spokeswoman for the White House said the soil in the garden had lead concentrations of 93 parts per million of lead. Health experts say it is safe to raise leafy vegetables in soil with concentrations of 10-50 parts per million, and urban gardens typically have raised lead levels. However, it is advised for young children to be tested for exposure to lead if they play in areas where lead concentrations exceed 100 parts per million. The Environmental Protection Agency puts the threshold for dangerous lead levels at 300 parts per million.

But even though lead levels in the first garden are far below that danger zone, the disclosure is awkward for a White House which has made prominent use of the vegetable garden to define Michelle Obama's role as First Lady,and to encourage sensible eating habits in children.

Children are especially vulnerable to exposure to lead, which can cause neurological and kidney damage, and stunt their growth.

The vegetable garden was an important symbolic break with the George Bush presidency, and it became a cause for environmentalists and the organic food movement in America who had urged the Obamas to use the White House to set an example of healthy eating.

Michelle Obama invited dozens of 10- and 11-year olds from a state elementary school in a transitional neighbourhood of Washington to the White House last March to help her dig up a 1,100 square foot plot of land near her daughters' swing set. Photographers were let in to take pictures of her kneeling in the dirt and wielding garden tools.

The first lady gave interviews joking about how all the members of the first famly would be required to weed on occasion.

As the weeks went on, and the White House garden grew, it became central to Michelle Obama's efforts to rebrand herself, and banish any residual damage from the rightwing attacks of the election campaign when she was cast as the stereotypical angry black woman. The White House featured blog posts on the garden's progress.

The school children were invited back to tend the plot and just two weeks ago to bring in the first harvest: 73 lbs of lettuce, 12 lbs of snap peas and one cucumber. Obama and the children then trooped into the White House kitchen to wash lettuce and shell and cook the peas for lunch, which they ate outside on red and white checked tablecloths.

Sam Kass, who followed the Obamas from Chicago as their personal chef, gave a short speech praising the gardeners for getting their yeild without resort to fertiliser or herbicides, and for using green compost.

The White House said the garden would go on. "The garden recently underwent extensive soil testing that proved it is completely safe," Katie McCormick Lelyveld, the first lady's spokeswoman, said. A lead level of 92 parts per million is significantly better than the government standard for a garden like this. The White House kitchen garden team is committed to producing fresh, safe and healthy food as a learning opportunities (sic) about health eating, and they'll continue to do so," Lelyveld said.

The White House would not say whether the Obamas or the children who had helped tend the garden would be tested for lead exposure.