LONDON - Starting four days of testimony about the sway of Rupert Murdoch’s newspapers over public life here, Gordon Brown, the former prime minister, on Monday denied Murdoch’s depiction of some of the most contentious exchanges between the two men and accused a Murdoch newspaper, The Sun, of undermining Britain’s war effort in Afghanistan.

He also rejected assertions by a former editor of The Sun that Brown’s wife, Sarah, had approved a story in 2006 on the medical condition of the Browns’ infant child, who had been diagnosed at 4 months of age with cystic fibrosis.