GLASGOW, Nov 10 (Reuters) - Six major automakers on Wednesday will commit to phasing out the production of fossil-fuel vehicles around the world by 2040, as part of global efforts to cut carbon emissions, the British government said in a statement. But sources familiar with the pledge"s contents said some big carmakers including the world"s top two, Toyota Motor Corp (7203.T) and Volkswagen AG (VOWG_p.DE), and crucial car markets China, the United States and Germany have not signed up. That highlighted the challenges that remain in shifting to a zero-emission future.Cars, trucks, ships, buses and planes account for about a quarter of all global carbon emissions, data from the International Energy Agency showed, of which the bulk comes from road vehicles. Sweden"s Volvo (VOLVb.ST), U.S. automakers Ford Motor Co (F.N) and General Motors Co (GM.N), Daimler AG"s (DAIGn.DE) Mercedes-Benz, China"s BYD Co Ltd (002594.SZ) and Jaguar Land Rover, a unit of India"s Tata Motors Ltd (TAMO.NS), were set to sign the pledge at climate talks in Glasgow, the latest initiative to help cap global warming by mid-century.Volvo has already committed to going fully electric by 2030. read more Britain, which is hosting the COP26 summit, said four new countries including New Zealand and Poland were joining other nations already committed to ensuring all new cars and vans are zero emission by 2040 or earlier. The statement comes on a day dedicated to transport at the conference. But the apparent unwillingness of China, the world"s largest car market, and the United States - the world"s largest economy and second-largest car market - to join the pledge raises questions about its effectiveness. GM said it is "proud to now stand alongside other companies, governments and civil society organizations to support the declaration to commit to working towards a transition to 100% zero emission vehicles by 2035." Ford confirmed its participation and said: "It will take everyone working together to be successful." Sources said that while the United States is not joining the pledge, key car-buying states like California and New York have signed up. An auto industry source said some carmakers are wary of the pledge because it commits them to a costly shift in technology, but lacks a similar commitment from governments to ensure that the necessary charging and grid infrastructure would be built to support electric vehicles. In the summer, the European Commission proposed an effective ban on fossil-fuel vehicles by 2035, accompanied by a commitment to charging infrastructure that carmakers had demanded. read more The world"s No. 4 carmaker, Stellantis (STLA.MI), was also missing from the latest pledge, as were Japanese carmakers Honda Motor Co Ltd (7267.T) and Nissan Motor Co Ltd(7201.T); Germany"s BMW (BMWG.DE) and Korea"s Hyundai Motor Co (005380.KS). According to sources, ride-hailing company Uber Technologies Inc (UBER.N) will also be a signatory. The British government statement said a joint declaration would also be signed by companies including food retailer Sainsbury"s (SBRY.L) and cities around the world aiming to make their vehicle fleets green. Reporting by Simon Jessop, William James and Nick Carey; Additional reporting by David Shepardson in Washington; Editing by Cynthia Osterman and Peter Cooney Our Standards: The Thomson Reuters Trust Principles.