Officials at the Oval cricket ground are on high alert after militant dads threatened to turn the outfield purple in a dramatic acid attack on the fifth Test.

The plot by members of the radical Fathers4Justice group was reportedly exposed by a cricket-loving father who wanted no disruption of the final Test match of the Ashes series between England and Australia.

Security at the Kia Oval ground in south London has been beefed up in response to the threat and Scotland Yard was investigating.

Surrey CCC chief executive Richard Gould said: "We want to make sure we can protect the pitch and the game. The security was stepped up so the Test can go on without any problems."

Fathers4Justice has a record of pulling off eye-catching stunts as part of its campaign for better access to their children for divorced or separated fathers.

According to an inside informant who leaked details of the Oval plot, acid was to be smuggled into the ground in children's drinks bottles. A noisy protest was planned at the same time as a distraction.

A carnival atmosphere is expected at the Oval on Saturday, with England set to celebrate retaining the Ashes against Australia. The final Test match could see captain Alastair Cook's men notch up a record 4-0 scoreline. The third Test match ended in a draw.

Fathers4Justice hit the headlines in June by attacking a painting in the National Gallery. Paul Manning, 57, was charged with criminal damage for allegedly plastering a sticker of a child on to the The Hay Wain by John Constable. No permanent damage was caused.

The group is waging war on the legal system which it claims discriminates against fathers in child custody cases. Fathers4Justice's website declares that the group is on a mission to "to end fatherlessness and the cancer of family breakdown" and reform family courts.