Viking River Cruises has changed its commercial agreements for 2014 in an attempt to stop agents from “blatantly undercutting one another”.

The line, which pays a base commission of 10%, is looking to stop agents advertising discounts on its products.

A Viking spokesman said: “We are asking agents not to promote openly the discounts they might offer, to help create a level playing field for all agents.”

Neil Barclay, head of sales at Viking, admitted that while he would prefer agents not to give away their commission, he could not stamp out discounting completely.

He told Travel Weekly: “We can’t stop them discounting, but we have a one‑price policy so customers get the same price whether they book through an agent or direct. This is helping agents by creating a level playing field.

“A lot of agents are blatantly undercutting each other in newspapers and online. It devalues the product.”

Barclay wouldn’t be drawn on how Viking would respond to agents that breached the new commercial terms.

He said agents who wanted to promote discounts to closed-user groups rather than to the general public would be supported by Viking. But observers said such groups could still be quite large.

Agents broadly welcomed the move. Phil Nuttall, managing director of Rivercruising.co.uk, said: “I support this decision 100%. This is an issue that has damaged the ocean cruise industry.

“Some agents are paying to advertise online that they are discounting – if that is your method for getting customers then you need to change your career.”

Chas Simmons, director of CruiseXperts, said the policy was “great in theory” but would never work. “How many lines have tried this? It doesn’t work and it can’t be policed,” he said.

Ocean cruise lines have incentivised agents not to discount. However, lawyers say pricing rules may breach competition laws.

Farina Azam, a solicitor for Travlaw, said operators cannot prohibit agents from offering discounts and cannot favour agents who do not discount.