From 18th February to mid June, Guilford Street will be one-way westbound from Grays Inn Road to Grenville Street for the replacement of a 32" gas main. Eastbound traffic will be diverted via Theobalds Road and Roseberry Avenue but cyclists will be better off using Tavistock Place and Ampton Street (CS6). Note that Guilford Street is already permanently one-way westbound west of Grenville Street. At the same time, a raised junction will be installed at Guilford Place.
Update 22/5/19: Works now extended till end May and new closure due to work on the Great Ormond Street Hospital building. As well as a new gas main, works include electricity and water.
Update 5/6/19: Works complete as far as Guilford Place but still one-way from there to Grenville Street.