Firefighters have been taking unofficial industrial action in a row over a pay deal intended to end the long-running fire service dispute.
Many fire stations across the UK are refusing to do any work other than answer 999 calls.
Anger has been mounting among firefighters since it emerged they would not receive a 7% pay rise from this month.
Employers say the rise, due to come into effect on Friday, is conditional on changes being made in the fire service which are not yet complete.
The two sides are due to meet to discuss the issue on Wednesday - which, as Bonfire Night, is one of the busiest nights of the year for fire crews.
Scottish counties including Strathclyde, Grampian, Fife
Across Northern Ireland
Parts of Cleveland
The prime minister's official spokesman said strikes were not the way forward.
He pointed out that the settlement the firefighters had agreed to was above inflation, and urged it to modernise.
Action has been taken in Cardiff and much of Scotland and Northern Ireland.
At least 13 counties in England have also been affected, including Tyne and Wear, South Yorkshire and Humberside, Bedfordshire, Wiltshire, Norfolk, and Nottinghamshire.
Local authority employers have offered an initial 3.5% increase, with a further 3.5% rise backdated to 7 November once the changes are complete.
Fire Brigades' Union leader Andy Gilchrist said his members were "extremely angry" their pay rise was to be paid in two stages.
The move was being seen as "political vindictiveness," he said.
"This has put fire service morale at an all-time low."
But the local authority employers said the agreement had always been that changes had to be made, and then verified by the Audit Commission - a process not expected until March.
They blamed union leaders for misleading their members.
"There's been no rabbits pulled out of the hat here, everybody knew this had to be verified before payment could be made," said Jeff Ord, president of the Chief Fire Officers Association.
Tristan Ashby, of the Retained Firefighters' Union which represents several thousand part-time firefighters, blamed the FBU for dragging its heels on modernisation.
"It has always been clear the deal was conditional, but they've done nothing about it," he said.
He said the four main conditions which should have been implemented were:
- An end to a ban on overtime
- Accepting "co-respondency", in which fire crews can keep people alive with defibrillators until ambulance crews arrive
- Allowing full-time firefighters to take on retained duties on their days off
- Allowing "mixed crews" of both retained and full-time firefighters.
But BBC correspondent John Morrison, at Glenrothes fire station in Fife, said firefighters there were "bitter" because they had been willing to make changes.
"They say they were in no way impeding new working practices, in fact they signed up to them a long time ago - and they say they are being penalised for something they are not doing", he said.
The FBU's Andy Gilchrist met the employers informally on Tuesday evening to lay out the parameters of Wednesday's talks.
BBC labour affairs correspondent Stephen Cape said the meeting was likely to be "long and tough", with Mr Gilchrist urging the employers to "fundamentally rethink" their position.
The employers, on the other hand, simply wanted to clear up misunderstandings and make their position clear, he said.
The FBU executive was also meeting to consider its next steps.
One member told BBC News there were likely to be calls at the meeting for firefighters to be balloted on industrial action.
The long-running pay dispute appeared to have been resolved in June, after months of strikes in which the Army used green goddess vehicles to provide cover.
Under the June agreement, 4% was paid immediately, back-dated to last November.
A further 7% was due this month, followed by a final rise of 4.2% next summer, lifting a
firefighter's pay to £25,000.