By Sarah Young

LONDON (Reuters) -Britain has selected Rolls-Royce SMR to build the country's first small modular nuclear reactors (SMR) as part of its plan to speed up the decarbonisation of the power network from the mid-2030s.

The government on Tuesday pledged 2.5 billion pounds ($3.4 billion) for the SMR programme over the next four years, aiming to kickstart one of Europe's first small-scale nuclear industries.

SMRs are typically the size of two football pitches, with parts that can be built in a factory, making them quicker and cheaper than traditional plants, which take more than a decade to construct and face planning delays in the UK.

The government also said on Tuesday it would invest 14.2 billion pounds to build a large scale nuclear plant, Sizewell C, in eastern England, as part of "the biggest nuclear rollout for a generation".

Rolls-Royce SMR, majority owned by FTSE 100 engineer Rolls-Royce which makes the power systems for Britain's nuclear submarines, said it would build three units.

"Doing lots of them gives you that opportunity to bring down the cost, that's the big prospect," Energy Minister Ed Miliband told Sky News. "It's huge for energy security, but it's also a huge opportunity for Britain."

Dozens of countries across the world, including the United States, Canada, Romania and Czech Republic, are looking at SMRs, opening up a big market if the British project is successful.

Rolls-Royce CEO Tufan Erginbilgic said he expected Rolls-Royce SMR to grow "materially".

Britain's state-owned energy company, Great British Energy - Nuclear, will aim to sign a contract with Rolls-Royce SMR and pick a site later this year, subject to regulatory approval.

The SMRs could support 3,000 jobs and power about 3 million homes once they are connected to the grid in the mid-2030s, the statement added.

Rolls-Royce SMR was chosen over Westinghouse, Holtec Britain and GE-Hitachi Nuclear Energy - an alliance between General Electric Co and Japan's Hitachi Ltd - in a two-year competition for the SMR contract.

($1 = 0.7396 pounds)

(Reporting by Sarah Young; Editing by Sachin Ravikumar and Paul Sandle)