CH2M has approached Atkins over the possibility of a merger, according to a report in The Times.
CH2M is understood to have contacted Atkins at the end of last year over the merger.
The Times reported that the approach came at a senior level but said it was unclear how far talks developed between the two firms.
Any merger would lead to the creation of a global engineering powerhouse, with Atkins boasting turnover of £1.9bn and CH2M’s latest turnover totalling £4.3bn.
CH2M and Atkins have a track record of working together on major construction programmes.
In March 2016 a joint venture between the two firms and Spanish engineer Sener scooped the £350m engineering delivery partner role for phase one of HS2.
The firms also worked closely on the Olympics and are currently acting as lead designers on the £1.5bn A14 Cambridge to Huntingdon upgrade programme.
The news comes less than a week after HS2 appointed CH2M’s European managing director Mark Thurston as its new chief executive.
Both firms are currently bidding for HS2 phase two contracts, which are expected to be awarded within the next couple of weeks.
CH2M is up against Bechtel and a joint venture between Mace and Turner & Townsend for the £170m development partner role, while an Atkins / Arcadis JV is bidding for the £350m professional services contract.
A spokesman for Atkins said: “It is our policy not to comment on rumour and speculation.”