It claims the Government disguised information to prevent the figure, three times previous ministerial estimates, from emerging.
In December, the junior health minister, Tom Sackville, told the Commons 5,000 nurses had lost their positions between 1988 and 1992.
However Unison says its analysis of Department of Health statisics shows the number to be 17,000, including more than 7,000 student posts.
The report, Lies Damn Lies and the Department of Health, said: 'It is clear that the massive reduction of nurses is contributing to lower quality and quantity care for patients in London which will be compounded by any further rationalisation as a result of the Tomlinson recommendations.
Stewart Barber, author of the report, said for the past six months ministers had explained the reduction in nursing staff as the transfer of senior nurses to managerial grades. However, of the 10,502 job losses in the Thames region only 1,225 nurses took up up senior posts.
Mr Barber said London represented 56 per cent of job losses nationwide.
Labour's health spokeswoman, Dawn Primarolo, said: 'The Government is effectively cutting the number of jobs by over 70 per cent in coming years. The real figures have not been revealed because they show the extent of their cutbacks in London's health services.
The Department of Health said that under NHS changes a number of senior nurses had been reclassified as managers and trainee nurses were no longer included in the numbers because of changes in the way they were taught. 'There has been a slight decrease and those two reasons account for pretty much all of it.