Transport Scotland has awarded the first of three multi-million pound design contracts for the A9 dualling programme to the joint venture CH2MHill/Fairhurst.

The three contracts are valued at £40m to £60m each, and this latest one includes the dualling of a 43km-long stretch of the A9 between Glengarry and Dalraddy.

Under the agreement, the joint venture design consultancy will start work on the middle section of the route in May, subject to a mandatory standstill period.

"The appointment of the first design consultancy for this mammoth project marks another significant milestone in this government’s commitment to the project."

Transport minister Keith Brown said that the dualling programme is the most expensive transport project in Scotland’s history and presents some of the most challenging work ever witnessed.

"The appointment of the first design consultancy for this mammoth project marks another significant milestone in this government’s commitment to the project," Brown said. "[We are] the first government to commit to dual this vital route between Perth and Inverness.

"As well as delivering faster and more reliable journey times and road safety improvements, when complete the design work offers the prospect of steady work for many years ahead for the three successful contractors."

Subject to the mandatory standstill period, Transport Scotland will also award the ground investigation contract for the 8km Birnam to Tay Crossing section to Soil Engineering. Work is expected to begin next month and will last for six months.

The A9 route between Perth and Inverness will be completely dualled with a total investment of £3bn by 2025.

The total project will include 12 sections, with three of these expected to be ready in 2017.