Direct mail users advised to switch to Royal Mail rivals - but questions remain on whether rivals can cope

Consultancies are advising brands to change suppliers to one of the five competitors: TNT Post, UK Mail, Citipost, DHL and UPS.

"We're telling clients that by switching you can avoid ‘upstream' delay and instead get ‘downstream' access, where other suppliers pre-sort the mail and deliver it direct into Royal Mail's Inbound Mail Centres, from where post moves more quickly for the ‘final mile' delivery," said Jonathan DeCarteret, founder of postal consultant Post-Switch.com.

Another postal consultant, OnePost, is telling clients to bring scheduled mail campaigns forward four or five days ahead to cope with delivery delays.

However newspapers reported last night that most rivals are already running at close to capacity, and said there was a strong chance they will not be able to cope with the expected flood of new business.

Guy Buswell, the chief executive of UK Mail which daily handles 17 million items of largely transactional mail, told the Daily Telegraph: "All parcel delivery companies run at close to capacity. That's how the business model works. A sudden influx of volume could be dangerous. We don't want to promise to take on a huge amounts of new work and jeopardise our existing clients. We all have to be very careful."

The most high profile brand so far to announce its defection from Royal Mail is retail giant John Lewis for its online deliveries. "We have ensured that postal strikes don't affect our customers by switching the business to other carriers," the company said.

The CWU said it had no choice but to announce a strike after the Royal Mail rejected its latest set of proposals, which Royal Mail described as "in reality a series of fresh demands".

Among the CWU's demands are that Royal Mail reveal its business plan, a pledge that modernisation will be agreed rather than imposed by management, a benefits package to reward postal workers for delivering success, and an independent inquiry into "bullying and harassment".

The 120,000 postal workers in the union last week voted three to one for nationwide industrial action.

Graham Cooper, managing director at OnePost, believes consumer opinion may play a larger role in sorting the current national strike than the last one in 2007. "The mood is different this time," he said. "I sense that public opinion isn't quite so behind the CWU as last time.  Lots of us have been through a difficult trading time and have had to bear the effects of modernisation and restructuring."