Londoners should not be "optimistic" that the Underground station staff dispute will be resolved before February, a union source with extensive experience of negotiations on the network told IBTimes UK.
"My impression is that the RMT representatives feel the [24-hour] strike [in January] was very strong and they will have support for members for more action... so I wouldn't be optimistic about a settlement," the source said.
The word of caution comes as the TSSA, the RMT and Underground bosses meet at mediation service Acas on Monday morning (16 January) for further talks.
"We are hoping to make progress towards a resolution which ensures that our Tube is both safe and adequately staffed," said Manuel Cortes, general secretary of the TSSA.
The RMT's ruling body, its national executive, agreed to call escalated strike action from Monday 6 February unless the union's demands are met.
The warning follows a walkout on 8 and 9 January which saw a majority of Zone 1 stations being closed, causing travel chaos across the capital.
The unions and Underground bosses agree that more staff need to be recruited after former Mayor of London Boris Johnson's Fit for Future stations programme saw hundreds of job losses and ticket office closures.
But the parties are in dispute over how big the recruitment drive should be.
Labour's Mayor of London Sadiq Khan branded the January strike as "unnecessary" and stressed that he had commissioned the independent London Travel Watch to look into the matter.
"We have concluded that much more needs to be done to make the new system as passenger friendly as it needs to be," said Stephen Locke, chair of Travel Watch.
"Our research has identified significant issues that must still be addressed, including the layout of stations, the services provided by ticket vending machines, staff training and availability, and assistance for people with disabilities.
"The new arrangements could, potentially, deliver good customer service – but we have listed several recommendations that need to be implemented before the reforms can be given a clean bill of health."
Labour leader Jeremy Corbyn intervened in the dispute last Thursday when he urged Khan to re-open ticket offices in some stations. Corbyn, who has represented Islington North in the House of Commons since 1983, told BBC London: "I do think there is a need to have a ticket office, particularly in the big interchange stations, Heathrow for example."
Steve Griffiths, chief operating officer for London Underground, said the company had "always intended" to review staffing levels. "We agree that we need more staff in our stations and have already started to recruit 200 extra staff and this is likely to increase further as we work through the other areas that need to be addressed," he added.
"Taking into account existing vacancies and natural turnover this means that over 600 staff will be recruited for stations this year. There will also be increased opportunities for promotion.
"All of this will ensure that our customers feel safe, fully supported and able to access the right assistance in our stations at all times. We encourage the unions to continue working with us on this process and the only way to resolve this dispute is to keep talking about how to improve our stations."