Wiltshire Council has appointed contractor MJ Church to take part in the next phase of plans to upgrade the M4 Junction 17, advancing a scheme officials say is intended to ease congestion and prepare for future traffic growth.

Cabinet members approved the move, agreeing that MJ Church will enter an Early Contractor Involvement (ECI) stage. Under that arrangement the contractor will work with the council to develop a build programme, identify potential cost savings and produce more accurate construction estimates to support a Full Business Case that will be submitted to the Department for Transport (DfT) in autumn 2026.

The upgrade proposals would complete signalisation of the roundabout at J17, widen carriageways on all approaches (including the M4 off‑slips, A350, A429 and B4122), increase the number of lanes across the motorway bridges from two to three, widen the circulatory carriageway and remove or reconfigure several nearby laybys. The council says the package has been designed to fit within the existing highway boundary and avoid land acquisition.

M4 Junction 17 improvements overview

The contract was put out to tender in October and Wiltshire Council received tenders from three bidders: MJ Church, Alun Griffiths and Octavius Infrastructure. The contract value for the ECI phase was advertised at £27M including VAT.

The council secured Outline Business Case approval from the DfT in spring 2024 and is preparing a Full Business Case, using information from the ECI stage to firm up costs and delivery plans.

If the DfT approves the Full Business Case and provides the expected funding, work could begin in spring 2027 and take around a year. Wiltshire Council estimates the total cost at about £31M, of which 85% would be met by the DfT and 15% by the council.

M4 Junction 17 links the motorway to the A350 corridor and provides access to towns including Chippenham, Melksham, Trowbridge and Westbury. The A350 has assumed growing importance as a north–south route between the Midlands and the south coast, and council documents say the scheme is intended to maximise junction capacity within existing land limits to avoid the need for piecemeal, sequential improvements as local developments proceed.

Before construction can start, the DfT must sign off the Full Business Case, based on outputs from the ECI phase. The council’s timetable envisages that period of design, cost-testing and approvals taking until late 2026, followed by procurement of the main construction contract and mobilisation ahead of the proposed 2027 start.

Martin Smith, Wiltshire Council cabinet member for highways, said: “The appointment of a contractor marks an important milestone for the M4 J17 project, which will benefit residents, businesses and road users for many years to come.

“Junction 17 is a critical gateway into our county, and these improvements will help ensure it can support the growth and economic activity we expect across Wiltshire.

“Appointing a contractor at this stage gives us the expert advice we need to strengthen our business case, before moving to construction if funding is confirmed by DfT.”

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