Drivers faced long delays on one of Britain's busiest motorways on Friday after a fathers' rights protester dressed as Batman climbed onto a gantry.
A Highways Agency spokesman said three of the six M25 anti-clockwise lanes were closed in the area around the gantry between junctions 14 and 15 near Heathrow Airport.
"Traffic has slowed down back to junction 17, but it is still moving," a spokesman said. "It is slower than normal."
All the lanes on the clockwise section of the motorway remained open, although a 40mph speed limit was in place. The agency reported delays of 90 minutes in the area.
Fathers 4 Justice, which stages high-profile stunts to campaign for divorced fathers to have greater access to their children, said the protester is thought to be one of its members.
In a statement, the group said the man, aged 48, from Farnborough, Hampshire, has not seen his eight-year-old daughter for a year.
Live television pictures showed the man standing on a gantry above the motorway next to a sign for Heathrow Airport as police watched from below.
He told Sky News by telephone that he had simply climbed up a ladder at the side of the gantry.
The stretch of motorway is close to the M4 junction and a few miles from the M3. It is one of the busiest stretches of road in the country.
A Metropolitan Police spokesman said officers were on the scene, but would not say what action they planned to take.
It is the latest in a series of stunts apparently organised by Fathers 4 Justice. Last month, two protesters climbed onto the roof of Labour Deputy Leader Harriet Harman's London home.
Campaigners have also climbed to the top of St Paul's Cathedral, the Houses of Parliament and Buckingham Palace.
In 2004, protesters threw flour-filled condoms at the then Prime Minister Tony Blair in the House of Commons.