By Rory Cellan-Jones BBC business correspondent

The Rail Maritime and Transport Union (RMT) is due to announce its new general secretary - but who is the likeliest candidate?

The union is only number 16 in the list of Britain's biggest but as recent weeks have shown it can punch far above its weight.

The man unveiled on Wednesday evening will have enormous power to disrupt the lives of commuters.

Even within the union movement there has been disquiet at the prospect of a victory for a hardliner

The likely winner is the assistant general secretary Bob Crow, a member of the Socialist Alliance, and viewed by many in the rail industry as the leading mover in the current wave of unrest.

If he does beat Phil Bialyk, seen as more moderate, and the outsider Raymond Spry-Shute, then both main rail unions will have young, left-wing general secretaries.

The train drivers' union Aslef is led by Mick Rix, a former member of the Arthur Scargill's Socialist Labour Party.

Recent weeks have seen industrial action at South West Trains, Arriva and ScotRail and there have been fears that the disputes could merge into a national strike.

Pay dispute

While stoppages at South West Trains have been suspended pending talks with the company, the RMT plans walkouts at Arriva on 1 and 2 March in a dispute over guards' pay.

Jimmy Knapp died last year

If victory in the battle to replace Jimmy Knapp, who died last year, does go to Mr Crow, rail industry managers will not be laying out the red carpet.

"His objective is to look for problems, not solutions," says one former senior executive who has clashed with him across the negotiating table.

'Confrontational approach'

In Whitehall too there is disquiet. One government source says that even though acting general secretary Vernon Hince has nominally been in charge, it is Mr Crow who has driven the current campaign of industrial action.

He draws a contrast between the RMT official and Aslef's Mick Rix, who has returned to the Labour Party.

Rank and file members may be less enthusiastic about strikes

"Rix is a shrewd operator, and more intelligent than Crow, who always sees confrontation as the first step."

Even within the union movement there has been disquiet at the prospect of a victory for a hardliner.

The Trades Union Congress (TUC) was embarrassed when an internal document describing Mr Crow as a "fundamentalist" who would "spell trouble for the government" was leaked.

But his supporters say all Mr Crow has done is voice the anger of thousands of rail workers who have seen their industry devastated by privatisation.

They accuse the government and the union establishment of trying to rubbish a man.

The rail unions argue that the fragmentation of the industry has made reaching agreements much more difficult.

They want a return to national pay bargaining.

While the Left is in the ascendant in rail union elections, there are some signs that rank and file members may be losing their enthusiasm for strike action.

South West Trains believes last week's decision to suspend strike action followed pressure from members unwilling to lose more days' pay.