The Enquirer understands Westbury will quit at the end of next month.
He joined O’Rourke in 2014 from Buro Happold where he was Chief Executive Officer.
Westbury was part of Laing O’Rourke’s Group Executive Committee reporting to Chief executive Ray O’Rourke.
He had global responsibility for the technical and engineering functions, including the Engineering Excellence Group.
Westbury said: “Approaching my five-year anniversary at Laing O’Rourke, I reflected on the terrific team in place and the way our technical and offsite-manufacturing operating models were being sought-out by clients for their major projects.
“With Laing O’Rourke achieving its three-year refinancing earlier this year, signalling the completion of a significant turnaround, it’s time for me to look to new challenges in the industry.”
Ray O’Rourke said: “Paul has spoken to me of his desire to share our vision of a modernised construction industry in new sectors, possibly from the other side of the table.
“We wish him well and are well-positioned on our path to embody technical excellence and certainty in all we do – the core of our Deliver 2025 strategy.”