BP has increased its stake in the British solar venture Lightsource BP as it prepares to strike a deal to power its offices with renewable energy from next year.

The companies announced plans to set up a 50:50 joint venture almost two years after BP made its return to the solar market by snapping up a 43% stake in Lightsource for £200m.

BP will increase its stake in the company by buying new shares for an undisclosed price to help accelerate Lightsource BP’s solar power targets. It had hoped to grow its portfolio to 6GW of capacity by 2023 but plans to reach 10GW over the same timescale.

BP has used the UK solar startup to re-establish a presence in the solar sector after it backed out of the market in 2011. It plans to use more solar power in its own offices too.

Dev Sanyal, the boss of BP’s alternative energy division, said the oil company was in conversation with the venture to source solar energy to power BP’s global offices next year.

BP’s commodity traders will also sell more renewable energy from Lightsource BP projects to other companies that want to run on clean energy.

Sanyal said: “BP is committed to helping meet the world’s rapidly growing demand for low-carbon energy. Solar, which is predicted to increase by a factor of 10 by 2040, plays a key role in this energy transition. That is why we want to invest more in Lightsource BP and to deepen our partnership.”

Lightsource BP has grown its pipeline of solar projects almost 10-fold across nine new countries in the last two years by harnessing BP’s international networks and energy trading expertise.

The company has also been experimenting with ways to boost the financial rewards of solar power projects, including trials that use double-sided panels or help to balance the energy system at night.

Nick Boyle, the group chief executive of Lightsource BP, said: “Two years ago, we were in four countries and we’re now in 13. We had a global pipeline of 1.6GW and today it’s over 12GW. It’s that sort of additional momentum which has been facilitated [by BP].”

BP’s fresh investment in the solar company came as environmental lawyers prepared to take action against a multimillion-pound BP advertising campaign.

ClientEarth said the company was misleading the public with claims of being committed to a low-carbon future. The charity is calling for the ads to be banned unless they include a health warning that BP’s oil and gas spending is contributing to the climate crisis.