Defence chiefs announced in November that they would purchase the three ships which will be built, subject to approval, by BAE Systems.
Construction is scheduled to begin later this year at the Clyde shipyards in Glasgow.
On a visit today, Defence Minister Philip Dunne announced the signing of a contract to buy parts for the vessels, which will be used in anti-piracy and anti-smuggling operations in waters around the UK and abroad.
The contract will allow BAE Systems to begin work on parts such as the engines and gearboxes, which require a longer time to make and therefore have to be ordered in advance of the main shipbuilding programme, the MoD said.
Mr Dunne said: "This £20m is a significant milestone, critical to the successful delivery of these three vessels for the Royal Navy.
"The OPVs will not only provide an additional capability for the UK but also at its peak, the OPV programme will safeguard more than 800 vital skilled roles in the shipbuilding industry."
Mick Ord, managing director at BAE Systems Naval Ships, said: "This first investment towards the programme is great news.
"We're incredibly proud to work on a new class of vessels for the Royal Navy and have already begun establishing the highly skilled team that will design, build and deliver them."
In November the Clyde shipyards were selected over Portsmouth for the planned construction of a new generation of Type 26 frigates for the Royal Navy.
The deal would guarantee work at Govan and Scotstoun until 2034, but the contract is not due to be finalised until after September's referendum.