The chief executive of Acevo has warned of “concerted attacks” against Oxfam and the Trussell Trust, with letters to the Times, the Charity Commission and Acevo members accusing MPs of attempting to curtail charities’ rights to free speech.

He said: “The attacks on Oxfam and the Trussell Trust represent an indefensible attempt to shoot the messenger and ignore the message about serious poverty in the UK.”

In a letter to the Times newspaper today Sir Stephen Bubb described the outcry over Oxfam’s campaign as “fabricated” and as a “dispiriting display of self-serving politicking from certain MPs and commentators.”

He added: “What Oxfam have to say may make uncomfortable listening, but those who close their ears ought to be ashamed of themselves.”

Bubb has also written to the William Shawcross, chair of the Charity Commission, to voice his support for Oxfam after a Conservative MP filed a complaint with the Charity Commission about the charity’s latest campaign last week.

He said: “It is essential that the traditional right, indeed duty, of a charity to campaign is guaranteed by the regulator. We have seen various attacks on this role in recent times, notably the Lobbying Act, and for many of us in the sector we worry that our right to be a voice for the voiceless and to champion the vulnerable through strong and edgy campaigns could be undermined.”

Writing to Acevo members he warned that “a vocal minority within Parliament and the press would like to see charities prevented from making any criticism of government policy” and that there had been “concerted attacks” on Oxfam.

He urged members to resist pressure to reduce campaigning activity and report any attempts to pressure charities to Acevo.