Hundreds of thousands of Tube commuters are set to find out whether to expect delays caused by staff going on strike.
Industrial action by Tube drivers is likely to cause much disruption
The Rail, Maritime and Transport (RMT) union will announce the results of a ballot of 1,800 maintenance staff at Tube infrastructure company Metronet.
Rail disruption is also expected in south England because of maintenance work at London Waterloo at Christmas.
South West Trains services from Surrey, Hampshire and the West Country are likely to be affected.
The Tube row is over jobs and pensions for maintenance staff, but it could mean 1,500 drivers work to rule because the lines will not be properly maintained during the strike.
There is a possibility of a series of one-day strikes in the run-up to Christmas.
The RMT's dispute is with Metronet's plans to allow its subcontractor Bombardier to maintain new Victoria and Circle Line trains, which the union said will "further fragment" the Tube infrastructure.
Metronet said plans to use Bombardier to supply and maintain new trains have been known about for a long time.
It accused the RMT of "an unflattering record of calling for industrial action in the month of December" - seven times since 1996.
But the RMT also said London Underground has not assured them proper track inspections will be carried out during a strike.
So drivers are being balloted on taking action "short of a strike" - which could mean refusing to work on safety grounds or driving very slowly which would disrupt services on 10 lines.
RMT leader Bob Crow told BBC London Metronet was not taking their concerns seriously.
"We are not in dispute with the travelling public and it is with deep regret that there would be any action," he said.
"But we are an organisation that represents railway workers and Tube workers - that's our job that we have to do."