Portside

LONDON, April 29 (Reuters) - Millions of commuters struggled to reach work on Tuesday as London Underground London Underground staged their second 48-hour strike this year to protest at job cuts and office closures in a dispute that is expected to cost businesses millions of pounds.

The walk-out that started at 2030 GMT Monday reduced the Tube network to limited services on nine out of 11 lines with extra buses and river boat services laid on to help the 3 million people who use the service daily get to work.

"It's unacceptable that millions of people are having their lives disrupted by today's Tube strike in London," Prime Minister David Cameron said on Twitter.

Transport for London (TfL), which runs London's public transport network, said thousands of staff and volunteers turned out on Tuesday to assist commuters, some of whom resorted to running, cycling or walking to get to work.

The walk-out, and a planned three-day strike next week, is over plans to close about 250 ticket offices and cut 950 jobs in a restructuring that TfL says could save 50 million pounds a year.

The Rail, Maritime and Transport (RMT) says the cuts will risk safety and quality of services but TfL says less than 3 percent of journeys on the 151-year-old Tube network involve ticket offices as most passengers use electronic travel cards and it has guaranteed no one will be forced from a job.

Mike Brown, managing director of TfL unit London Underground, said staff were doing all they could to keep London moving.

The action follows a similar two-day strike in February which crippled the train network but after a second walkout planned for February was averted to let talks take place. Those negotiations broke down earlier this month.

BUSINESS COST

The Federation of Small Businesses estimated that strike cost small businesses, which make up about 99 percent of London companies, about 600 million pounds ($1 billion) in lost working hours, business and productivity.

Its estimate of the financial impact of the strike was based on a poll of its members that found businesses took a hit of about 1,297 pounds each due to cancelled meetings, staff absences, and difficulty transporting good and services.

"Many businesses will be rightly concerned about the potential impact five days will have," the federation's national chairman John Allan said in a statement, adding that businesses should draft contingency plans for the next action from May 5.

Commuters setting off for work found the services were not all running to plan, posting photos of packed train platforms.

"Dear London tube commuters, please mind the gap between the timetable and reality," tweeted Kerli Peetsalu.

Road were busy with extra cyclists, taxis and hire cars, some offering special deals to lure tube-stranded commuters

"Ah, that magical time when the roads are filled with cyclists who haven't been on a bike since the 90s," tweeted Maxim Lutkin, while other posted photos of themselves at home, abandoning work for the day.

Unlike in February, the walk-out this week is just by the RMT union with three other unions representing rail workers not involved, having accepted a promise of no forced redundancies and that no supervisors would have to reapply for their own job.

RMT Acting General Secretary Mick Cash said London Underground had refused to budge over the cuts, which, he said, would risk safety and damage quality of service. He has blamed rail management for the failure of the talks.

Trade union experts said the success of former RMT leader Bob Crow, who died suddenly on March 11, in extracting concessions from employers through hard talk and industrial disruption has set the mould for those vying to replace him.

Simon Walker, director-general of the Institute of Directors, said unions cannot be opponents of modernisation.

"The RMT must accept that the Underground is not exempt from the need to increase efficiency and promote value for money for taxpayers and customers," he said in a statement. ($1 = 0.5948 British Pounds) (Editing by Alison Williams)