General secretary of the Trades Union Congress John Monks is going to 'look into and possibly review' his membership of a health scheme guaranteeing treatment outside the NHS, according to a union spokesman.
His payments to the Simplyhealth friendly society have emerged as the TUC challenges the government on the private sector's role within public services.
For£22.50 per month, members can get a bed in a network of 50 hospitals outside the public sector.
Mr Monks has been a vocal campaigner against increased private sector involvement in the health service, challenging the prime minister on the private finance initiative publicly on a number of occasions since the election, as well as during a visit to Downing Street.
The union's annual conference next month is set to see widespread opposition to an enhanced role for the private sector.
Simplyhealth, which changed its name from Manor House Healthcare last year, has had a lengthy relationship with the trade unions. Until it was sold in 1999, members of the friendly society could opt for treatment in the society's own Manor House Hospital.
A spokesman for the Transport and General Workers Union said its general secretary, Bill Morris , has 'cancelled his membership in March this year'. He added that Mr Morris was 'furious' at the decision to close down Manor House Hospital.
Spokesman for Simplyhealth Michael Evans said: 'There are a number of people who are members of unions who are not waiting to be treated by the NHS. They are using the society to trigger treatment that they could be waiting 18 months for otherwise.'