Two 24-hour London Underground strikes will bring further travel chaos to the capital this week as workers plan to walk out again over proposals for a four-day week.Rail, Maritime and Transport union (RMT) members are planning industrial action for all of tomorrow and again on Thursday, threatening huge disruption to Tube services.Talks will be held today in a last-ditch bid to avert the strikes which are in response to issues including the new shorter week which the union has said it disagrees with.Transport for London (TfL) told passengers to check before they travel if the strikes go ahead, warning of disruption to services during, before and after the action.The conciliation service Acas will be involved in today's talks between RMT and TfL over the new arrangements, which the drivers' union Aslef has previously accepted.But an RMT source insisted Tube drivers had 'very real' concerns about spending longer hours in the driver cab, becoming more fatigued, losing flexibility around their shifts and the impact that could have on a role where 'safety has to come first'.TfL - which hopes to run at least half of Tube services on the strike days - has said it would never compromise safety, adding that the new shift system was voluntary.The Circle and Piccadilly lines will be suspended, as will the Metropolitan between Baker Street and Aldgate; and the Central between White City and Liverpool Street.Services will start late and finish early, with no service before 6.30am or after 9pm – and some disruption is also expected on the mornings of Wednesday and Friday.London Overground, Elizabeth line, Docklands Light Railway, buses and trams will continue operating during the strike periods but are expected to be far busier.Claire Mann, TfL's chief operating officer, said: 'We are disappointed to see the RMT continue with this industrial action.'We still believe that the points they have raised can be worked out in time, through more detailed discussions and we are continuing to talk to the union's representatives to find a way to avoid disruption to London.'She added: 'Our proposals are, and have always been, clear. The completely voluntary four-day week has been designed to improve work-life balance and any of our Tube drivers who do not wish to take up the new proposed way of working and associated changes to working arrangements can remain on a five-day working pattern.'A significant number of drivers have indicated that they want us to progress plans for the pilot of this new working pattern on the Bakerloo line, bringing benefits both for our colleagues and our customers.'We urge the RMT to work with us so we can resolve this dispute. In the meantime, we are asking customers to check before they travel and allow plenty of extra time for their journeys.'An RMT spokesperson said: 'Our members have raised serious concerns around fatigue, longer shifts, reduced flexibility and the impact these proposals could have in a safety-critical role.'RMT remains available for meaningful talks, but London Underground cannot push ahead with changes to drivers' working conditions while refusing to properly address legitimate safety and workplace concerns.'The strikes are officially due to take place from 0.01am until 11.59pm tomorrow and again on Thursday.Ed Richardson from the BusinessLDN group said: 'These strikes are yet again set to make it more difficult for Londoners and visitors to make journeys around the city.'Although the city did not come to a complete standstill during the April strikes, with the Overground and Elizabeth line running and many Londoners turning to other modes of transport like e-bikes and buses. Staff at Farringdon station direct commuters to an alternative entrance near the Barbican due to overcrowding on the Elizabeth line during the last RMT Tube strike in London on April 23'But for many businesses that rely on people visiting in person, the impact of these strikes – whether they go ahead or are called off at the last-minute – will have already been felt through cancelled bookings and people changing their plans.'We urge both sides to reach a sustainable agreement to put an end to the damaging uncertainty hanging over businesses and London's economy.'The RMT held its last Tube strike during the week of April 20, during which TfL ran around half of all its services.TfL said that on every day of that industrial action, more than half of normal demand was observed on the Tube, while Friday April 24, the final day of action, the network saw 94 per cent of normal demand.Another RMT strike planned to begin on May 18 was called off at the last-minute after the union claimed TfL had 'shifted its position', allowing it to 'explore our members' concerns' about new rosters.Last week, the Daily Mail revealed militant union barons were plotting a 'winter of discontent' in a bid to get inflation-busting pay hikes.Union sources said barons were gearing up for national action which could shut down hospitals, schools and the railways after Labour made it easier to call strikes.The RMT is threatening a UK-wide strike campaign to get above-inflation pay rises - although the union is not expected to launch any ballots until after the summer.Eddie Dempsey, the union's boss, wrote to all his members in April saying he was demanding blanket pay rises higher than the retail price index (RPI) level of inflation with no-strings attached.When he wrote to members RPI was more than 4 per cent, while the officially preferred consumer price index rate was at 3.3 per cent. The latter has since dropped to 2.8 per cent but experts say it could rise above 4 per cent again later this year.