The Royal Mail has admitted encouraging managers to spy on striking staff, as the unofficial postal action continues.
Thousands of postboxes have been sealed in London
The firm said it was "only right" to try to collect evidence of activities provoking unlawful industrial action.
But the postal union said the tactics were an "absolute disgrace", as talks resumed on Saturday to try to end the dispute.
Deliveries continue to be seriously affected in London and elsewhere, with post boxes sealed up in the capital.
About 25,000 workers are now on unofficial strike action across the capital and in more than a dozen regional centres.
However, there are hopes the talks in a London hotel between management and union negotiators could lead to staff going back to work on Monday.
'Return to work' document
The BBC's John Moylan, at the talks, said: "They're going through points one by one, to try to get this form of words, this 'return to work' document that both sides are happy to sign up to.
WHERE IS THE DISRUPTION?
London services are extremely disrupted, with most post boxes sealed and people advised not to post letters
Special Delivery services suspended in London
Other areas affected include: Chelmsford, Colchester, Coventry, Maidstone, Milton Keynes, Oxford, Portsmouth, Slough, Southend, Stoke-on-Trent and Swindon
"And then talks on more substantive issues to do with moving to a single post, and more flexible working arrangements up and down the country, they can then be taken on by talks at Acas on Monday."
The Royal Mail said there was "nothing sinister" about its actions in encouraging local managers to give informaiton about strike action.
A source at the firm told BBC News the company had been "inundated" with material showing the strikes had been organised.
It wanted to gather information with a view to taking legal action, but it was too early to consider such action at the moment, the source said.
The Communication Workers Union (CWU) said the Royal Mail's actions suggested it did not want to resolve the dispute.
"It sounds like they just want to make
political capital and bully and intimidate their workforce," general secretary Billy Hayes told the BBC
Radio 4 Today programme.
Royal Mail chairman Allan Leighton urged both sides to end the "war of words", and focus on the issues at the heart of the dispute instead.
"The real issue is there's a bit of a battle between the management and the unions as to how the company's going to be modernised," he told BBC News.
"My view on this has always been very straightforward - we want to run a modern company with a modern union, working together to create the best postal service in the world."
Business leaders have pleaded with ministers to step in to the increasingly bitter dispute, but the government says the management and unions must resolve the issue between them.
"Neither megaphone diplomacy or unofficial action will solve the
problem," said Trade and Industry Secretary Patricia Hewitt.
"Both sides need to put all efforts into working... to reach an
agreement and stop this damaging action."
But shadow trade secretary Tim Yeo said the government should do more to warn both sides that the action is being "uniquely damaging".
"There's so many alternatives to using the letter post, unlike previous strikes," he told BBC Radio 4's Today programme.
"This strike will result not just in a temporary loss of business for Royal Mail but a permanent loss."
Royal Mail's competitors are already being allowed to take over more deliveries on a temporary and limited basis.
Tesco has switched to private firm Parcelnet to deliver books ordered on its internet site.
Subjects up for discussion on Saturday include a national agreement on overtime, and the implementation of a single delivery service in London.
The CWU is not officially condoning the wildcat strike action, but is negotiating on behalf of the workers.
There have already been three days of talks between management and the union negotiators, but have so far ended without agreement.
The CWU says the reason for the action is that Royal Mail managers have been trying to force through changes to working practices without agreement.
General secretary Billy Hayes attacked the Royal Mail for not negotiating seriously, saying it had sent only a "very low level manager" into the talks.
The dispute is due to be taken to conciliation service Acas on Monday.
The action began in London about two weeks ago.
Thousands of post boxes in the capital have now been sealed up, and millions of letters are caught in the backlog.
Postal workers in Chelmsford, Colchester, Coventry, Maidstone, Milton Keynes, Oxford, Portsmouth, Slough, Southend and Swindon had joined the walkouts by Friday night.
The Passport Office has been flooded with inquiries from holidaymakers concerned about documents held up in the post.