Linda Gould, 42, has not received a single official payment for her 18-year-old son, Toby, who is in full-time education, since her husband left her last November after 22 years of marriage.
Formerly a self-employed gardener, Simon Gould, 40, has worked for the Territorials for up to seven days a week since the separation and is currently serving in The Falkland Islands.
He is thought to be receiving almost £20,000 per year, but Mrs Gould, an administrator for Riverford Organics, can't claim support as her husband is receiving expenses, not a salary.
A letter from the agency in May said: 'The Territorial Army is classed as a Special Occupation and Child Support Regulations state any payment made in respect of the performance of duties as a member of any territorial or reserve forces is not to be treated as earnings.'
Mrs Gould was keen to stress that she had no argument with her estranged husband, who has made two voluntary payments since the split, but with the agency's rules.
'This is all wrong as it's sending out a message to anyone who wants to avoid paying child support that all they have to do is sign up to the TA and they'll be exempt,' she said.
Angered by the situation, Mr Steen has written to the work and pensions minister, Lord Hunt, and tabled questions in the House of Commons. 'This is an absurd and shameful situation,' he said.
'I do not believe it is parliament's intention that those in full-time employment with the Territorial Army, earning almost £20,000 a year, should be exempt from their responsibilities to their children.
'The CSA had been incredibly slow and dilatory in the handling of this claim. It is such an absurd story that I intend to make an absurd fuss about it.'