UPDATE 2.45pm: The RMT has followed the TSSA union in suspending strike action after an improved offer from Network Rail.
The looming Network Strike threatens not only to derail travellers' plans to get around the country, but even those looking to go abroad after Heathrow Express announced it would run during shortened operating hours on Monday 25 and Tuesday 26 May.
If the strike goes ahead, Gatwick Express will also be badly affected and only able to run an "extremely limited train service" on the affected days.
Days of negotiations between union bosses and rail chiefs have made little progress, making the prospect of travel chaos next week increasingly likely.
The high-speed train to Gatwick airport will be completely shut down on Tuesday after running a reduced service on Monday. Those with an early flight are also advised that some services may be disrupted on Wednesday morning.
Heathrow Express, the direct service from Paddington in Central London to the international airport, will be less affected with trains running once every 15 minutes from 5.10am to 18.30pm on Monday and 7.30am to 18.30pm on Tuesday.
Heathrow Connect, which stops at a number of suburban stations along the way, will not be running at all.
Keith Harding, the operations director for Heathrow Express, told operations customers travelling on Monday and Tuesday to keep an ear out for further updates:
Check before you travel. You can get the latest information on our website and Twitter account. Things are developing. When we know more, we will let you know.