Fears have been raised about the future of the programme to build the RAF's next high-tech fighter amid disarray over Labour's defence spending plans.The Tempest Global Combat Air Programme (GCAP) is a joint venture between the UK, Italy and Japan to build a sixth-generation stealth jet, with a proposed target of 2035 for it to enter service.The first concept aircraft, a supersonic testbed for new technologies, had been earmarked to take its first flight by the end of next year, before prototypes and production follow.The 'combat air demonstrator' is currently being built at BAE Systems in Lancashire. But the Ministry of Defence is now silent on whether the December 2027 deadline is still in place, amid growing confusion and concern about the wider Defence Investment Plan.Liberal Democrat defence spokesman James MacCleary tabled a written question to the MOD in May asking about the deadline, but despite it being required to respond by June 2 he has yet to receive a reply. The MOD has also not responded to a request from the Daily Mail for clarity. Labour ministers are embroiled in a major row over the scale of the DIP, with expectations that the Treasury wants to limit the spending boost to less than half the £28billion experts believe is required.Defence Secretary John Healey stunned Westminster this morning by resigning with a blast at the money on offer. According to reports, Japan is concerned by Britain's failure to deliver on an overdue £6 billion investment in GCAP.