Transport for London (TfL) offered trade unions the promise of a four-day working week in return for calling off November's planned Tube strikes.
A letter from Nick Dent, the TfL director, to the Aslef trade union on Tuesday pledged to "set out a proposal for delivering an average four-day working week".
The condition for opening discussions on a four-day working week was accepting a 3.8 per cent pay rise and calling off "all pending industrial action". Aslef's strikes, which had been planned for Nov 7 and Nov 12, were suspended that day.
In January, Tube workers were given a five per cent pay rise by Sadiq Khan, the Mayor of London, that cost the taxpayer £30 million, prompting accusations that he had found a "magic money tree".
The Conservatives called the deal a "total capitulation" to the unions, and questioned how much the plans would cost London taxpayers.
Tube drivers agreed to accept a pay rise that will take their salaries to just shy of £70,000 per year, and after the new Labour Government offered public sector workers pay deals worth about £10 billion.
Tube drivers currently work a five-day, 35-hour week. The four-day week plan would see no overall change in working hours, but sources said that in practice it would mean longer working days.
Underground drivers also enjoy 43 days holiday a year, thanks to a previous deal that saw time spent on shift but not working rolled up into extra holidays.
Walkouts by the RMT, the other Tube drivers' union, were suspended last week after shop stewards said they had secured a "significantly improved offer" on pay and working conditions. No details of that offer have yet emerged into the public domain.
TfL's letter to Finn Brennan, the Aslef London organiser, seen by The Telegraph, said: "By January 2025 we will set out a proposal for delivering an average four-day working week with a paid meal relief included in working hours, which means fewer hours at work, whilst improving the reliability and efficiency of our service and maintaining the current 35-hour contractual working week."
Aslef called the proposal a "genuinely groundbreaking agreement" and claimed it would lead to drivers working fewer hours. The union said: "In every four-week pay period, you will be working 10 hours and four days fewer. This also means an increase in the hourly rate of pay."
Keith Prince, City Hall Conservatives' transport spokesman, said: "A four-day week is great for drivers, but Tubes run every day of the week - meaning that TfL, and ultimately taxpayers, have to pick up the bill for more staff. If this was the cost of stopping the strikes, then it's total capitulation. What will they give them next time?"
A TfL spokesman said: "We have committed to delivering a proposal to our unions in January of how a four-day working week might work.
"As with any proposal, there are still details that need to be worked through and any changes would need to be mutually beneficial, preserving or improving the reliability of our service for our customers and improving efficiency. We will continue discussions with our unions."