Addison Lee, the UK private car hire company owned by private equity giant Carlyle, is buying rival Tristar.
The financial details of the deal were not disclosed, but City sources put the valuation at around £30m.
Tristar, which operates the largest luxury car fleet in the UK, has revenues of £50m per year.
Addison Lee chief executive Andy Boland said:
There will be significant growth in the executive car market following the 15 per cent jump in volumes we saw in 2015. With Tristar, we will be able to take our service to the next level and into new markets around the world.
The combined fleet with have around 1,000 cars. Tristar currently has 200 staff and 450 drivers.
The deal is expected to add to Addison Lee's transatlantic business, which City A.M. previously reported as a high priority for the company as it fights against upstart rivals Uber, Hailo, and Lyft.
Boland told City A.M. earlier this year Addison Lee was aiming to provide a quality service and would not just compete with the likes of Uber on price.
Tristar Worldwide was backed by private equity house Octopus Ventures in 2008 as part of a £9.75m management buyout, to which Octopus contributed £4.8m.