An electric fence could be set up to shield Big Ben from terror attacks and publicity stunts.

Commons authorities have ordered officials to look into the feasibility of running an electric current through a new, stronger fence along Bridge Street next to Big Ben.

This comes as the US uncovered a terror plot targeting its major financial institutions.

A Commons source told The Times: "The fence would give you a nasty shock and make you jump off, but would be at a height where if you feel off, it wouldn't kill you."

Bridge Street, which links Westminster Bridge with Parliament Square, was the entry point for two Green Peace activists who scaled Big Ben in protest against the war in Iraq. At the moment, an ornate iron railing on a stone wall surrounds the perimeter of the House of Commons.

Wider measures are also said to be discussed. Roads could be closed off to prevent a car bomb from being driven too close. Booms in the River Thames could thwart an attack from the water.

A temporary security screen on the Strangers' Gallery could be replaced by a permanent one. To do this, the September sitting of the House of Commons could be scrapped.

The current screen was erected after a flour bomb attack on MPs earlier this year.

Meanwhile, Britain maintains "heightened readiness" against terrorism, a Home Office spokesperson said. Measures to protect Britons both at home and abroad are "under constant review".