Royal Mail says the backlog of post is now down to five million items
The Communication Workers Union (CWU) plans to go to the High Court to stop Royal Mail using agency staff to clear the post backlog caused by the strikes.
Royal Mail had planned to hire 30,000 temporary workers to deal with the backlog of post and the Christmas rush, which it insists complies with the law.
The CWU hopes to show that the company is breaking employment law.
Meanwhile, Royal Mail and union leaders are continuing a second day of talks at the TUC aimed at averting more strikes.
Royal Mail and the union executive were briefed by their negotiating teams late afternoon, with talks set to continue into the evening.
The CWU has announced that their executive will meet to discuss what has been put on the table early Wednesday afternoon.
They will then decide whether to continue with the new strikes or call them off.
If no deal on pay and working conditions is reached, up to 120,000 CWU members will stage three 24-hour strikes from Thursday.
'In line with the law'
"We are taking legal advice and are assessing the evidence," a CWU spokeswoman said.
"We are expecting a decision later today."
The law states that firms cannot recruit workers to do the job of employees called out on a legitimate strike.
WHAT DOES THE LAW SAY?
The law states that it is a criminal offence for agencies to provide workers to do the job of employees who are out on strike
It is also potentially a criminal offence for the end employer - but the offence would be aiding and abetting the agency
However, it is only a criminal offence if the temporary workers are employed by the agency and not by the company directly
If the agency merely supplies the workers and the company employs them directly then the company can deploy them as it pleases, employment lawyers say
In any case, Royal Mail's argument is that these workers will not be doing the work of staff who are on strike, rather they will clear any backlog of post and deal with the Christmas rush
Royal Mail says its recruitment is "fully in line with all employment law".
"The recruitment is not to bring people in to do our postmen's work when they are out on strike," the company said in a statement last week.
It insists its action is designed "to make sure that we have people to help clear any backlogs between strikes as well as to help - as happens every year - with the seasonal build up of mail in the run up to Christmas".
Royal Mail told the BBC it had nothing further to add to last week's statement on the issue.
Royal Mail expects its backlog to have fallen from 30 million to five million items since last week's strikes.
The company said employees returned to work on Saturday to start clearing the piles of letters and parcels delayed by two 24-hour walkouts last Thursday and Friday.
The CWU had claimed the backlog was in excess of 100 million items on Saturday.
Neither Royal Mail nor the CWU would comment after Monday's talks but TUC general secretary Brendan Barber said "we have had useful discussions".
CWU deputy general secretary Dave Ward said the dispute was "fundamentally" about jobs but the issue of the Royal Mail's £10bn pension deficit also needed to be resolved urgently.
He said: "There's no prospect of us building a successful future for the Royal Mail and for the workforce unless the government actually deal with that issue."
The talks are taking place at the Trades Union Congress (TUC), which helped to resolve a dispute between the two sides in 2007.
If this week's strikes go ahead, they are due to involve:
• Thursday - 43,700 staff in mail centres, delivery units in mail centres, network logistic drivers and garage staff walking out from 0400 GMT
• Friday - 400 workers at three sites in Plymouth, Stockport and Stoke, who assist mail centres by reading and entering mail addresses
• Saturday - 77,000 delivery and collection staff across the UK.