RMT general secretary Bob Crow has written to London Mayor Ken Livingstone and LUL managing director Tim O'Toole urging them to block the sale.

"Metronet's desire to offload the only purpose-built train overhaul and modification unit in the southeast shows once again that the privateers are more interested in making money than in providing a service," Bob Crow said today at RMT's annual general meeting in Exeter.

"Acton Works has been maintaining Tube trains since the 1930s and remains vital to the efficient and safe running of the network.

"It was Acton Works that came to the rescue after the Central Line derailment a couple of years ago, modifying 3,000 motors to get the service running again, and which worked round the clock to modify brackets on District Line trains that had been causing motor problems.

"Metronet claim that falling work streams and the optimistic expectation of the arrival of new rolling stock in 2007 and 2008 justify the sale.

"But not only is there is no credible evidence that workloads should be falling, we have also been hearing alarming reports that Metronet may be deliberately scaling down repairs on the existing stock to save money in anticipation that the new trains will arrive on time.

"The sale would mean even more fragmentation of the Tube's infrastructure and further sub-contracting of essential maintenance and repair work with the cost of yet another privateer's profits being passed on to taxpayers and commuters who are already paying through the nose.

"Any attempt to make our members pay for this sale with their jobs will be resisted, with industrial action if necessary," Bob Crow said.

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Note to editors:The sale of the Acton Works in the run-up to the part-privatisation of the Tube was blocked in the late 1990s after a campaign which included demonstrations outside the office of the deputy prime minister.