Royal Mail is sealing post boxes in London to prevent people adding to the huge backlog of mail caused by unofficial strike action.
Wildcat strikes started in London two weeks ago
About 25,000 postal workers are now thought to be taking part in the "wild cat" strikes at 15 of the 73 postal centres across the country, said Royal Mail.
A second day of talks at the company's headquarters in London broke up on Friday night without agreement but will begin again on Saturday.
Meanwhile Royal Mail's competitors are being allowed to temporarily take over deliveries in an attempt to begin to clear the backlog of millions of undelivered mail.
On Friday Postal services regulator Postcomm relaxed delivery rules to give four private companies with licences permission to deliver letters for their own customers during the strike.
A Royal Mail spokeswoman told BBC News Online most of London's 13,000 postboxes would be sealed up by the weekend.
The unofficial action began in the capital almost two weeks ago, in the wake of a 24-hour official strike over London weighting payments.
Now nearly half of London's 202 delivery offices and two thirds of its postmen and women are taking part in the strike.
As talks broke up for the night, Communication Workers Union Deputy General Secretary Dave Ward described the meeting between union leaders and management as "constructive."
But later he reacted in anger to accusations by Royal Mail that staff had been intimidated into taking part in the unofficial action.
Roger Baynes, director of operations for Royal Mail South East, who has been heading negotiations, said he hoped the dispute could be resolved before they went to the conciliation service Acas on Monday.
Speaking on Channel 4 News he warned staff were being intimidated into joining the action.
"The vast majority of our employees don't want to be out on strike, they
don't want to lose money, they don't want to disrupt services to our
"But you have picket lines and they are scared for those reasons."
But Mr Ward told the BBC: "There is no truth whatsover in the suggestion that bullying and harassment is the major sticking point in the dispute."
He called for an independent investigation into any bullying allegations.
Instead he said the issues were working conditions and one delivery time.
The CWU has said the row is not about London weighting, but about attempts to force through changes in working conditions.
Staff in Cambridge, Swindon and Stoke-on-Trent went out on unofficial strike on Friday while in Filton, Bristol, workers staged a one-hour walkout at lunchtime.
WHERE IS THE DISRUPTION?
London services are extremely disrupted and people are advised not to post letters
Special Delivery services suspended in London
Areas still affected are Swindon, Slough, Gravesend, Maidstone, Essex, Milton Keynes, Coventry,
Oxford, Portsmouth, Warrington, Rugby, Stansted airport, Chelmsford, Colchester,
Milton Keynes, Coventry, Stoke-on-Trent, Cambridge and parts of Berkshire, Lanarkshire and Hampshire have also seen walkouts
Special Delivery services - where customers pay extra to ensure mail arrives - are suspended in London.
The Royal Mail is advising customers not to post letters in central London, while elsewhere it recommends avoiding affected areas.
The Passport Office has been flooded with inquiries from holidaymakers concerned about documents held up in the post.
The Home Office says one-year restricted passports can only be issued without birth certificates and paperwork in exceptional circumstances.
Travellers can go to passport offices for more expensive fast track applications.
About 16 million items of mail every day are becoming stuck in the backlog in London.
Some businesses have had no mail for 10 days, and executives say they have lost tens of millions of pounds.