The Royal Mail said it preferred the current voluntary arrangement
The Bank of England and the Royal Mail are two of 57 new organisations which will soon have to treat Welsh on an equal footing with English.
Heritage Minister Rhodri Glyn Thomas is to bring the organisations under the Welsh Language Act, meaning they will have to provide some services in Welsh.
It is the largest number of bodies ever placed under the act simultaneously.
The move comes as the assembly government prepares a bid for further powers over the Welsh language.
At present, 426 public bodies, such as the health service, already have to provide bilingual services.
The new organisations will be obliged to produce a language scheme showing how they would provide services in Welsh for the public.
A spokesperson for the Royal Mail Group said it was "fully supportive of the Welsh Language Act.
"We are proud of our achievements in relation to the voluntary Welsh Language Scheme which this year celebrated its 10th anniversary.
"We remain fully committed to providing a comprehensive bilingual service to our customers and will continue to do so either in a voluntary or statutory capacity," the spokesperson added.
"Like most businesses, we would have preferred the voluntary arrangement that has delivered this good service all these years.
"However, we do not expect the new arrangements to cause us any significant problems at all."
Other bodies being brought under the act will be the Competition Commission, UK Sport, Ofcom, the British Council and the new Olympic Delivery Authority.
According to the assembly government, the move is about keeping up with the times.
The act at present only compels public bodies to offer Welsh services
Mr Thomas said: "This is part of our commitment to ensure that there are more services available to the public in Welsh."
"We are currently preparing a legislative competence order (LCO) in order to ask Westminster to devolve powers to the assembly to legislate in the field of the Welsh language and I will make a further announcement shortly."
"But it's important that we also use the powers that we already have to offer a choice of Welsh language and bilingual services to the public."
"The 1993 Welsh Language Act set firm foundations for developing bilingual services in Wales. By now 426 bodies have Statutory Welsh Language Schemes. It is now time to take a step forward."
However, private companies such as supermarkets and mobile phone operators do not have to abide by the rules, despite calls by language campaigners to change that.
The key judgement that is made by the minister when deciding whether an organisation can be brought under the act is that they "provide services of a public nature".
There is pressure for the new language powers bid to Westminster to include the ability for the assembly to compel private business to treat Welsh equally, if they provide such services.
A poll conducted by ICM for BBC Wales last summer found 63% of people questioned did not think the law should be changed to make businesses provide services in Welsh.