The Competition and Markets Authority said it was concerned that the deal will reduce much-needed additional competition for rail signalling supply just as major contracts are coming up for tender.
On HS2, the firms make up two of the four shortlisted bidders for the £540m job to design and buildsignalling systems that will control trains travelling at speeds of up to 360km/h and up to 25 years of technical support.
It added that the loss of competition across both mainline and urban signalling markets could increase costs for Network Rail and TfL and have an adverse knock-on effect on taxpayers and passengers.
Colin Raftery, Senior Mergers Director at the CMA, said: “Network Rail currently spends close to £1bn annually on mainline rail signalling – and this is expected to increase in future, as equipment needs to be replaced and the UK transitions to digital signalling.
“The cost of signalling, and