An ancient byelaw was responsible for a “conspicuous” bundle of straw hanging from London’s Millennium Bridge on Tuesday.

The ritual took place as work began on the installation of safety netting under the bridge, which is owned by City Bridge Foundation, a bridge owner responsible for five Thames crossings.

The “quirky” tradition is down to the Port of London’s Thames Byelaws, which state a “bundle of straw large enough to be conspicuous” must be suspended when the headroom of a bridge arch or span is reduced from its normal limits.

Robert Edeson, of City Bridge Foundation’s contractor, CAN Ltd, did the honours, lowering the straw over the edge of the structure and no doubt turning a few heads.

A City Bridge Foundation spokesperson said: “This is one of those quirky traditions London is famous for, but it also does serve a practical purpose, to warn shipping when the headroom under a bridge span is reduced.

“The bundle of straw is lowered by our contractor when they’re doing work under the bridge, in this case, to install netting ahead of work to replace the separation layer between the aluminium bridge deck and the steel structure underneath.

“As a 900-year-old charity which maintains five Thames crossings and is London’s biggest independent charity funder, we’re proud of the part we’ve played in the history of London and our modern-day role looking after some of the capital’s key transport infrastructure.”

The tradition comes from article 36.2 of the Port of London Thames Byelaws, which says: “When the headroom of an arch or span of a bridge is reduced from its usual limits but that arch or span is not closed to navigation, the person in control of the bridge must suspend from the centre of that arch or span by day a bundle of straw large enough to be conspicuous and by night a white light.”