Britain's private postal operators have made substantial inroads into Royal Mail's customer base as a result of the strikes at the end of the year, and any further upheaval will accelerate the process, warns a significant player in the industry.

London-based Post-Switch, a business that specialises in providing customers with the cheapest and most efficient mail options, says it helped about 45 companies change from state-owned Royal Mail to rivals over the past month.

"We are seeing a lot of people inviting us in to tender for the first time and only one of those that we help moved away to private operators has gone back to Royal Mail, despite the end of the industrial action," said James Campbell-Clause, Post-Switch sales and marketing director.

Clothes catalogue company Lands' End is one of the companies that has transferred its 10m items-a-year business to DHL and TNT. Managing director Tim Curtis said he felt compelled to safeguard his employees and cannot see himself going back. "It [the private alternative] is slightly cheaper and a good service. We would like to stay with Royal Mail, but cannot as long as this uncertainty remains," he added.

The shake-up follows a series of days of industrial action by members of the Communication Workers Union in recent months in protest at the way modernisation was being introduced into the state-owned postal operation. The strikes were called off in the run-up to Christmas while further talks between management and unions take place to try to find a permanent peace settlement.

Those talks are ongoing and many heavy users of the postal system, such as mail-order firms, continue to be nervous, fearing a second bout of disputes after the holiday period is over.

Royal Mail said it could not quantify at this stage what level of business had been lost as a result of the disputes and many private operators, such as TNT, have played down any advantages they've gained, given they use the state system to deliver the "final mile" under access agreements which continue with Royal Mail.

Private companies are also keen not to be seen to be benefiting from the upheaval at Royal Mail, which recently reported a first-half operating profit of £184m, but admitted it had a pension deficit of more than £10bn. A spokeswoman for the CWU said there were no hard figures on what post volumes had been lost by Royal Mail, adding: "There have been a lot of stories about customers such as Amazon moving away, but some of this has been incorrect. It is obviously all unhelpful [to us] but there is no firm, empirical evidence yet."

DX Group said it had won some extra volumes but expected much of it to migrate back gradually. "We were not in favour of the strike because we do not want the trend of e-substitution (customers moving from physical mail to email) to accelerate," said John Coghlan, chief executive of DX.

Post-Switch says customers are moving from Royal Mail because they want greater certainty of service, but others want lower prices. Campbell-Clause claims a mailout of 50,000 items with a two-day delivery could cost 21p using Royal Mail, but only 17.4p using TNT, according to its calculations.

"We found savings of 23% for Jo Malone, an Estée Lauder brand," he added, arguing that changes in VAT payments and "zonal tariffs" at Royal Mail will further play into the hands of private rivals.

TNT took the government to the European court over the VAT payments that are levied on private operators, but not on the majority of Royal Mail postal operations.

The Dutch-owned company has been promising to launch a door-to-door letters business across Britain, following successful trials in Liverpool, if it can get the government to shelve VAT on its deliveries. The Conservative Party is understood to have considered doing this, should it win the next election.

Private companies are already looking at new ways of delivering post with DX offering customers the chance to pick up mail from their local post office under an agreement with Post Office Counters.