Two lorries were involved in the accident which blocked all three lanes and the hard shoulder between junctions eight and nine shortly after 1415 BST.

No-one was seriously hurt but some casualties were taken to Hemel Hempstead Hospital with minor injuries.

Tailbacks reached back beyond the M25 at one stage and roads in nearby towns were gridlocked.

The motorway was reopened during the evening rush-hour but police were warning it would take some time for traffic to get back to normal.