Tarmac’s purchase of Welsh contractor Alun Griffiths is being investigated by the competitions watchdog.
The Competition and Markets Authority (CMA) has opened an investigation to assess the impact of the proposed deal.
It is now inviting any competition or public interest issues to be submitted by 25 May 2018.
Tarmac agreed the deal to acquire road surfacing specialist Alun Griffiths in January for an undisclosed fee.
Following the agreement, the materials giant announced that Alun Griffiths would continue to operate under its own name as part of the Tarmac business.
It also revealed that founder Alun Griffiths would continue to provide strategic advice, despite not being listed as a director of the business.
The investigation comes after the news last month that the CMA had asked Tarmac to address “competition concerns” over its deal to buy concrete plants from rival Breedon.
Alun Griffiths reported a £50m increase in revenue to £154m and doubled its pre-tax profit to £2.3m at its last accounts for the year to 31 December 2016.
In May it was named project developer on the £70m dualling of a major link road in Worcestershire.
The firm is also lined up to carry out ancillary works on the Swansea Tidal Lagoon should the project go ahead.
Earlier this month Tarmac moved to strengthen its infrastructure team by bringing in Carillion’s former UK civil engineering managing director to lead the division.
Tarmac and Alun Griffiths have been contacted for comment.