Firefighters yesterday began taking wildcat action, refusing to do more than answer 999 calls.
They are angry that that the second stage of an agreed 16% pay deal will be paid in two parts - 3.5% on Friday and another 3.5% next year, pending approval by the audit commission - rather than as a single 7% rise on Friday.
Local authority leaders have insisted the FBU knew that external verification was needed before the 7% could be paid in full.
However, firefighters made it clear that they were expecting the 7% to be paid in one go this week.
Action took place in Scotland, Norfolk, Northern Ireland, Humberside, Yorkshire, Gloucestershire, South Wales and Wiltshire.
Unofficial sources said that as many as 23 of the country's 56 fire brigades were hit by strike action yesterday.
The dispute threatens to unravel a pay deal agreed in June after a series of official fire strikes that crippled services across the country.
Andy Gilchrist, the general secretary of the FBU, and his deputy, Mike Fordham, were today meeting John Ransford, one of the employers' chief negotiators, and other management officials, in London.
Mr Gilchrist said: "This is seen as some sort of political vindictiveness, and on the frontline is seen as bad form. People are extremely angry." The FBU executive is due to meet tomorrow to decide on its next move.
One member of the executive yesterday said that the pay deal could now be ripped up, sending the dispute back to square one. He refused to rule out the possibility of a ballot for fresh industrial action.
Sir Jeremy Beecham, the chairman of the Local Government Association, which represents employers, told BBC Radio 4's Today programme: "The agreement is absolutely clear - the agreement that was reached in June.
"Seven per cent is payable from - not on - from November 7, subject to agreement being reached in respect of a variety of matters contained there. Good progress has been made on that. But it is also subject to verification of change by the audit commission.
"What the employers are saying is - and this has been clear for some time to the union - that the 7% would be backdated to 7 November. No one is going to lose out through this, providing the verification takes place," he said.
Mr Ransford said that firefighters would "get nothing" if they failed to "sign up to this stage of the proceedings".
"If there is disruptive industrial action, the employers will feel that the firefighters are not honouring their part of the bargain, and the deal which was put together could start to unravel," he warned.
Unofficial action did not appear to be as widespread today as it was yesterday.
Some firefighters who took action yesterday were working normally today, although others elected to join the protest.
Kenny Ross, the Scottish secretary of the FBU, described action today as "sporadic", with firefighters in Dumfries and Galloway joining the protest.
Humberside fire service said FBU members who took part in the wildcat action were working normally today. Fire crews in East Sussex joined the unofficial industrial action, and are now answering only emergency calls.
Five out of six full-time stations in the area last night began working to rule, leaving Eastbourne as the only one operating normally.
In a statement, Brighton and Hastings firefighters said they would respond to emergency calls, but would not participate in training and other duties. Firefighters in West Sussex and Kent have not yet joined the action.
