ExxonMobil, the US oil giant, could face embarrassment at its annual general meeting at the end of the month as a shareholder campaign to improve its board structure gathers momentum, with six large UK investors now signed up.

The campaign, which is led by the shareholder activist Bob Monks, is aimed at splitting the combined role of chairman and chief executive at the company - held by Rex Tillerson. Such a split is considered best practice in many markets, notably the UK, as it encourages independent oversight.