An award-winning charity is in turmoil after suspending its high-profile chief executive following a row over his active membership of a leftwing political group.

John Nicholson, chief executive of the UK Public Health Association (UKPHA) and an outspoken critic of government social policy, next week faces a disciplinary hearing instituted by senior officials of the charity.

UKPHA is an independent UK-wide voluntary association, formed in March 1999, of individuals and organisations committed to promoting public health.

Mr Nicholson is also a senior national officer with the Socialist Alliance, a group of old Labour voters. He faces allegations that the UKPHA was compromised because Mr Nicholson's home address is both the registration address for the UKPHA and of the Greater Manchester branch of the Socialist Alliance.

The chairman of the UKPHA, Geof Rayner, has claimed in a letter to Mr Nicholson that the dual registration put at risk the charity's neutrality and reputation and potentially undermined its fundraising abilities.

Mr Nicholson refutes the charges and claims that attempts to remove him from his post are politically motivated by pro-government council members of the charity. Supporters of Mr Nicholson say his suspension amounts to "political victimisation."

Under Mr Nicholson, UKPHA won the award for best new charity at the UK Charity Awards 2000, promoted by Charity Times. He was shortlisted in this year's awards, held on September 6, as charity manager of the year for his success in raising the charity's profile.

Although he was a member of health secretary Alan Milburn's modernisation action team, which helped draw up the NHS Plan in the spring of 2000, he is a persistent old Labour critic of government policies on public health, Aids, asylum seekers and the private finance initiative.

Leaked documents seen by SocietyGuardian.co.uk reveal that Mr Nicholson was also accused by the charity's chairman Geof Rayner of misrepresenting UKPHA policy. Mr Rayner says in the letter: "Not only have you failed to undertake external communications in the manner specified, you have sinced acted to fundamentally misrepresent the policies of the UKPHA."

The letter to Mr Nicholson also accuses him of failing to disclose his involvement with the Socialist Alliance - which Mr Rayner described as a "potential conflict of interest" - when he applied for the chief executive post over 18 months previously.

Mr Nicholson refutes the allegations, and claims in a letter to UKPHA council members that the disciplinary proceedings are motivated by officials who want "a much more cosy relationship with the Westminster government."

The letter states: "I believe that the chair and vice chair do not want true political neutrality...but that (probably without even realising this) they want to be uncritical of the Labour party (and even more so of the Labour government)."

Mr Nicholson says that the use of his home address to register the charity was cleared with the charity commission, which also said there was no problem with the address being shared by other organisations with which he was involved.

Mr Rayner today confirmed that a disciplinary hearing against Mr Nicholson would take place on Monday. However, he refused to comment on the grounds that he did not want to prejudice the outcome of next Monday's hearing. "I will make no comment on any allegations within the disciplinary process."

Mr Nicholson was unavailable for comment.