Striking Royal Mail staff have begun returning to work on the advice of their union and the employers.
The Communication Workers' Union (CWU) and Royal Mail finished more than 15 hours of negotiations at 0330 GMT on Monday, saying "an understanding" had been reached.
The CWU said the agreement was adequate for it to recommend to its members they return to work.
Staff in Coventry, Warrington, Hatfield, Chelmsford and parts of London have already returned to work.
A worker on a picket line in Greenford, west London, told BBC News the agreement was "fantastic news" and staff would return to work on Monday afternoon.
But as the strike was unofficial, some postal workers could choose to stay out despite the advice of their union.
Both sides will still go to scheduled talks at mediation service Acas on Monday.
In a joint statement they said: "We will be talking to our people today and will now go to Acas to resolve all outstanding issues relating to pay and major change.
WHERE IS THE DISRUPTION?
Staff in Coventry, Warrington, Hatfield, Chelmsford and parts of London returned to work on Monday morning
London services extremely disrupted, most post boxes sealed and people advised not to post letters
Special Delivery services suspended in London
Other areas affected include: Colchester, Maidstone, Milton Keynes, Oxford, Portsmouth, Slough, Southend, Stoke-on-Trent and Swindon
Guide to the affected areas
"We wish to apologise to customers for the huge inconvenience that has been caused and our first priority is to clear the backlog that has built up and get services back to normal."
After news of the breakthrough reached the picket line in Greenford, Kevin Dommet told BBC News: "It's fantastic. I'm glad they've got a formula for us to get back to work.
"We'll go back to work when we're instructed by our union officials."
He added that could be in the afternoon.
More than 20,000 workers in London and a dozen regional centres were on strike, with letterboxes sealed to prevent the backlog of undelivered letters growing.
The Royal Mail's chief executive Adam Crozier earlier warned that failure to modernise the post service could destroy the Royal Mail.
The company must turn itself around because it had lost £1.8bn over the last two years and faced increased competition, he told the BBC.
There has been some newspaper speculation that the government may intervene if the dispute drags on but on Sunday the Deputy Prime Minister John Prescott would not be drawn on the matter and said: "We're not party to the negotiations."
'Fit to compete'
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Mr Crozier said unofficial strikes cost Royal Mail 66,000 working days in 2001 and it was still considering legal action over the current walkouts.
He added: "We have to make provisions today that will allow us to be fit to compete for the next 10 or 20 years.
"We've got competition coming in very quickly from electronic media and also from the regulator which is bringing in other carriers.
"If we don't sort this out, if we don't modernise now, we won't have a future to think about."
CWU deputy general secretary Dave Ward said the causes of the dispute involved many offices and many local working practices.