LONDON (REUTERS) - Britain's government promised on Thursday to upgrade roads and carry out what it said was the biggest rail investment in more than 100 years as part of its strategy to get the economy growing again while also keeping a tight lid on spending.

A day after finance minister George Osborne announced cuts to government budgets worth 11.5 billion pounds (S$22.71 billion) in the 2015/16 fiscal year, his deputy Danny Alexander detailed the Conservative-led coalition's infrastructure plans.

Alexander described them as "the most comprehensive, ambitious and long-lasting capital investment plans this country has ever known."

Alexander announced 28 billion pounds would be spent by the government on improving roads from 2014 to 2020 and it would support 30 billion pounds in rail investments.

He also announced a multi-billion pound guarantee designed to help build a new nuclear power station at Hinkley Point and details of energy prices that the government will guarantee for offshore wind power developers.

The overall plan, worth a total of 100 billion pounds, put some flesh on the bones of 300 billion pounds in capital spending commitments set out until 2020.

The announcement did not amount to a new injection of cash as the government battles to eliminate a large budget deficit.

The Conservative party is seeking to set out its stall for the 2015 general election which is likely to be won or lost on how well the economy is performing.

Thursday's announcement aimed to draw in private sector cash by instilling confidence in the government's commitment to infrastructure.

The opposition Labour party, which has previously urged more short-term spending to revive the economy but has been wary about committing itself to increased borrowing, kept up its criticism of the government for doing too little on investment.

"When is the government going to pull its finger out and actually start to build some of these things?" said Chris Leslie, a Labour lawmaker and a party spokesman for economic issues.