Following the announcement that seven government departments have provisionally agreed to reduce their budgets by up to 10%, underground and business leaders are urging the Chancellor not cut funding for major tube projects.
They say schemes like the planned £250 million makeover of Holborn station are vital.
Holborn serves the 'midtown' business district but is closed several times during rush hour because it can't cope with its 100,000 passengers a day.
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.