LONDON, England (CNN) -- Postal workers in Britain began a second round of strikes Thursday, disrupting deliveries across the country and deepening concern about mail services over the Christmas season.
More than 43,000 postal staff across the United Kingdom were striking at mail centers and distribution units, according to Royal Mail and the Communication Workers Union, which represents the striking workers.
Some 400 people in three cities in England planned to strike Friday, and 77,000 delivery and collection staff were due to strike Saturday, the union said.
The action follows two consecutive days of nationwide strikes last week that resulted in limited collection and delivery of mail.
The union has been trying to reach agreement with Royal Mail on modernization plans, job security, and workloads. Union members say Royal Mail has not consulted the union on its plans to bring in changes such as new machines that might replace certain positions and lead to layoffs.
The union stepped up the threat of strikes this month after learning Royal Mail planned to hire some 30,000 temporary workers to cover any planned walkout.
"It possibly goes against U.K. employment law," said a union spokeswoman, who asked not to be named. "They have certainly broken the spirit of the law by employing these temporary agency staff."
She said this week's strikes went ahead after Royal Mail failed to respond to an agreement put forward by the union on Wednesday morning.
Asked about why the Royal Mail did not respond to the proposal, a spokeswoman said, "As far as we're concerned, we're actually still open to discussions. We're still available to talk. Certainly our door isn't closed."
About 2 million pieces of mail were backlogged as of Wednesday because of the strikes last week, the spokeswoman said. It is too soon to tell how much mail could be affected by this week's strikes, she said.
Royal Mail said services would be worst affected Thursday and again on Saturday, the day when postmen plan to strike. It said it had contingency plans in place to minimize the effects as much as possible.
"We are really sorry for the significant disruption industrial action by the CWU is having on customers and for the inconvenience these further stoppages will cause," the Royal Mail told customers on its Web site. "We are continuing to urge the CWU to call off all strike action and concentrate with us on supporting customers during the autumn and Christmas peak mail period."
There has been concern among retailers, especially those who rely heavily on direct mail, that strikes or their effects could last through Christmas.
TNT Post, a Royal Mail competitor, says its service is suffering because it still relies on the postal service for the final stage of delivery.
"There are regulatory barriers in the postal market which prevent TNT Post from providing a full end-to-end delivery of mail," it said in a statement this week. "The strikes show it is high time that these barriers are lifted in order for a real alternative service to Royal Mail's to be introduced."
The Direct Marketing Association said earlier this month, before the strikes took place, that British businesses would suffer a "severe body blow" in any strike.
Amazon.co.uk, the British arm of the popular online retailer, said it was doing everything it could to ensure that customers' orders are not disrupted by the strike, including using other carriers.
Other businesses are doing the same, and may even stick with those private carriers after the strike, according to the ISBA, which represents British advertisers.
"The strikes have really started to undermine the relationship our direct mail members have with Royal Mail and they are finding that alternative delivery services are become increasingly appealing," said David Ellison, marketing services manager for the ISBA. "Some members have looked at short-term contingency plans with these other delivery services, while others are considering transferring their business permanently."
One of those private carriers is TNT Express, a parcel service that is separate from TNT Post. It has seen a spike in business because of the work stoppage, said TNT Express spokesman Nick Murray.
"With parcels, we've seen approximately a 10 percent increase since last week, since the strikes commenced," he told CNN. "We've seen a lot of extra calls to our call centers -- around 20,000 extra calls."
Dave Ward, the deputy general secretary of the Communication Workers Union, said the agreement it proposed Wednesday included the promise of no strikes over Christmas so long as the Royal Mail stuck to the terms of the deal.