The dispute is over new rotas

BBC correspondent Andrew Winstanley said the union involved, the RMT, was willing to take the row "to the wire".

The RMT hoped the mayor would intervene but he asked them to return to talks and call off the 24-hour strike, due to run from 1830 GMT on Sunday.

It is also balloting members for action over allegations of safety breaches during the strike on New Year's Eve.

The dispute centres around new rotas, linked to a 35-hour working week deal which had been agreed with the Rail, Maritime and Transport (RMT) union.

As it is not proposed to implement the new rosters until 5 February, I can see no reason for short-circuiting the procedure for discussing these issues London mayor Ken Livingstone Ken's letter to the RMT Bob Crow's letter

But the union says there are plans to "displace" many more staff than had been agreed, and this would leave some stations dangerously understaffed, it claims.

It also fears job cuts and wants the new rotas, due to be implemented in February, to be delayed.

But in a letter to RMT general secretary Bob Crow, mayor Ken Livingstone said suggestions that jobs would be cut were "categorically untrue" and claims that new rotas would be unsafe were "factually false".

He asked Mr Crow to keep to the deal which it had described as "groundbreaking" last year.

"As I am informed, the RMT broke off negotiations and called these strikes before the procedures for settling outstanding issues on the rosters by negotiation had been exhausted.

'War with trade union'

"As it is not proposed to implement the new rosters until 5 February, I can see no reason for short-circuiting the procedure for discussing these issues."

But Mr Crow dismissed the letter as a "cheap publicity stunt", adding: "Those new rosters were not a part of the 35-hour week agreement we signed in good faith last year, and we do not seek to re-negotiate that deal."

Chief negotiator, Bobby Law, said it was "astounding" that a mayor who had pledged to stand "shoulder to shoulder" with RMT members in the past, was now involved in "orchestrating" a dispute with the union.

He added: "If they wish to continue in some kind of war with this trade union, we will certainly continue to defend our members jobs and conditions, for as long as it takes."

London Underground boss Tim O'Toole said on Friday they would only keep stations open on Monday where sufficiently qualified staff were available.

Meanwhile Mr Crow has written to Her Majesty's Railway Inspectorate, calling for an investigation into the alleged health and safety breaches during the first strike on New Year's Eve.

He claimed stations without staff were left open to the public and that unqualified staff were used.

RMT members will now be balloted on whether to take action short of a strike, probably a go-slow, over the alleged safety breaches.