Hidden field “ufprt” being added to Razor Umbraco Form – Why?

I have got a form (below) that is posted to an umbraco surface controller.

@using (Html.BeginUmbracoForm("AddToBasket", "Basket"))
{

<h1>@Model.productSelectionModel.Product.Title - @Model.productSelectionModel.Product.Price.ToString("C")</h1>

<ul>
    @foreach (var productOption in Model.productSelectionModel.ProductOptions)
    {
        <li>@productOption.Option.Title</li>
        @Html.DropDownList(productOption.Option.Id.ToString(), productOption.ValuesInOptions.ToSelectList(f => f.OptionValue.OptionValue1, 
                                                   f => f.Id.ToString(), 
                                                   "Select"));                                             
    }
</ul>

    <input type="submit" value="Add To Basket">
}

When I look at the HTML rendered for this form it seems to have added a hidden field called ufprt. Does any one know what this is? Why is it being added, I’m not using it any where ( I don’t think I am anyway)

Any ideas?

<input name='ufprt' type='hidden' value='6C01896EF3D5F430F9ED041DD2B0D31F89FA969A085C6F