The U.K. Defense Secretary said $129.8 million of investment has been awarded to BAE Systems to begin work on the next generation of Royal Navy submarines, Successor-class submarines, which will carry the U.K.’s strategic nuclear deterrent.

Two contracts worth $77 million and $52.5 million have been awarded to BAE Maritime-Submarines, which is leading the vessel design, the MoD said Dec. 16.

The next generation submarines will be the largest and most advanced boats operated by the Navy, and their design and construction will be the most technologically complex in the history of the United Kingdom, the ministry said.

The investment will allow BAE, which currently has more than a thousand people working on the Successor program, to begin work on some initial items for the submarines that are due to replace the Vanguard class from 2028.

It is essential these items, which include structural fittings, electrical equipment, castings and forgings are ordered now to ensure the submarines are able to meet their 2028 in-service date, the ministry said.

MoD has also released an image of how early designs are shaping up. The image is part of an update on the program that was presented to Parliament.

Defense Secretary Philip Hammond said: “This ($129 million) £79 million investment is another important milestone in our preparations to build these world-leading submarines. The current Vanguard class of deterrent submarines perform a vital role in the defense of the U.K. and the replacement for this capability is of national importance.”

Hammond added that as many as 850 British businesses could benefit from the supply chain.

First Sea Lord Admiral Sir George Zambellas said: “The Royal Navy has been operating continuous at-sea deterrent patrols for more than 40 years and the Successor submarines will allow us to do so with cutting-edge equipment well into the future. The submarines are being designed to be some of the stealthiest in the world and are expected to see operational service from the late 2020s right up to the 2060s.”

The Successor design and build program is among the most complex ever undertaken by British industry, MoD said. The total number of MoD and industrial staff currently working on the Successor program is around 2,000, with more than half working as engineers and designers.