The threat of more travel chaos for Tube travellers could become a reality as the biggest rail union, the RMT, decides to ballot its workers on strike action over pay.

Ballot papers will go out next week to 8,500 RMT tube workers and the first of what could be a series of 24-hour, or even 48-hour, stoppages could be staged in the first half of September.

RMT workers striking over safety concerns brought London Underground to a virtual halt last month when a 24-hour strike was held.

This latest dispute concerns an LU initial pay offer of 3.2% to which a no-strike clause was attached and which the RMT turned down.

'Absolute rubbish'

A 2.5% offer was then made, which was increased to 3.0%.

But this was again turned down, although LU said on Tuesday it would be bringing in the rise anyway.

General secretary Bob Crow said: "The company has been spinning the lie that we have turned down an increased pay offer.

"That is absolute rubbish. In fact the company reduced its offer when we refused to sign away our right to strike, and LU is refusing the chance to have this dispute settled by independent mediation.

"Mediation settled last year's pay round. What are they afraid of this year?

"In the last 10 years Tube staff levels have fallen by a third, while passenger numbers are up by 30% and passenger revenue has doubled.

"Our members deserve recognition that they work harder and carry far greater responsibility."

The executive committee of the train drivers union ASLEF is due to meet later on Wednesday and is also expected to call for a ballot over industrial action.