Oil executives face a second day of grilling by US lawmakers today over the Horizon Deepwater well rupture that has unleashed a huge oil slick in the Gulf of Mexico.

BP, Transocean and Halliburton are being interrogated for their roles in what could be the worst oil spill in US history.

Meanwhile, BP is involved in a race against time to plug the leak and contain the slick.

Troops and prisoners have also been involved in the effort to limit damage to the coast, where crude has started to reach shorelines.

Investors have driven the value of BP shares down more than $30bn, far exceeding even the worst estimates of the spill’s cost, Reuters reports.

In response to the oil spill, the US Government has also announced that the Minerals Management Service (MMS) will be divided so collection of oil royalties and safety inspection of offshore drilling are separated.

The MMS currently carries out both roles, which has drawn criticism from some US lawmakers and environmental groups.