The measure is being implemented because the Environment Agency placed the area into a "prolonged dry weather category", with low flow in the River Thames impacting the Farmoor Reservoir.
Thames Water noted that current reservoir levels at Farmoor are below average at 92 per cent, exacerbated by forecast high temperatures and increased water demand.
This decision follows the UK experiencing one of its warmest and driest springs in over a century in 2025 and England's warmest June on record, with similar hosepipe bans already in effect in Yorkshire, Kent, and Sussex.