<form action="<?php echo Mage::getUrl('abc_checkout/cart/placeorder'); ?>" method="post" id="confirm_order_form" class="info-static" onsubmit="return submitOrder();">
.
.
.
.
<div class="buttons-set">
<p class="back-link"><span onclick="history.go(-1);
return true;" class="engine-btn">Back</span></a></p>
<button type="submit" title="<?php echo $this->__('Place your order') ?>" class="engine-btn continue"><span><span><?php echo $this->__('Place your order') ?></span></span></button>
<div class="mt30">
<?php echo $this->__('Please be patient.'); ?>
</div>
</div>
</form>
I want to disable this submit button after one click on front end.