The Royal Mail has warned staff not to accept Christmas gifts valued at more than £30 for fear of bribery charges.
In guidance on its website, Royal Mail said although it is the "season of goodwill", staff needed to be careful not to "fall foul of the Bribery Act".
The new Bribery Act, which came into force on 1 July, made it illegal to offer or receive bribes, and to fail to prevent bribery.
Royal Mail said tips could be accepted, but not in return for favours.
The organisation's My Royal Mail website, which is designed for staff, said under the heading Christmas and the Bribery Act: "It's the season of goodwill. But it's important to be careful about accepting gifts so you don't fall foul of the Bribery Act."
"A bribe is not just about money, it could be any gift, hospitality or favour," it also said.
Under the heading "other guidelines", it advices staff to reject gifts worth more than £30.
"Gifts may only be given or accepted up to a value of £30; any gifts over £30 must be politely declined," it said.
Royal Mail spokesman Ian McKay said postmen and women have been able to accept Christmas tips for many years, but the company had to take the Bribery Act "very seriously".
"It is entirely up to customers if they wish to give a tip to postmen and women at Christmas and to what value," he said.
"Like all other major companies, we take the Bribery Act very seriously and have a duty to clearly communicate to all of our staff what it means for them.
"We have made it absolutely clear to our people that Christmas tips are allowed."
Before the new Bribery law, similar regulations dated back to 1906 but the Bribery Act also covers bribing a foreign public official and a corporate offence of failing to stop a bribe on behalf of your organisation.