The FTSE 100 company said it would not be able to afford the rising cost of its contribution to the pension plan from 2018.
The move will affect 90,000 members of the defined benefit scheme.
Unite will consider strike action if Royal Mail does not "respond positively", and the CWU said "unagreed" changes would face a ballot.
Royal Mail wants members of the scheme to change to a defined contribution plan - in which the company and staff contribute to a pension pot with no guarantee of how much the eventual payment will be.
The Royal Mail pension plan currently has a Â£1.7bn surplus but Royal Mail forecasts that it will run out in 2018 and that continuing contributions under the existing arrangement is "not affordable". It currently contributes Â£400m a year but forecasts this will rise to more than Â£1bn in 2018.
It said: "With our unions, we have been actively exploring possible changes to potentially enable us to keep the plan open on a defined benefit basis after March 2018 as part of our pension review process.
"We will continue discussions with our unions during and after the consultation. We will carefully consider feedback and any affordable proposals that members or their representatives make."
However, Unite said the consultation was a "cause for concern" and said it would look to mitigate the impact of any proposed changes during and after the consultation process.
Brian Scott, national officer of Unite said: "The consultation is complex and the company needs to ensure that its employees, our members, clearly understand the potential impact on them, and the reasons and justification for the proposed changes."
He added: "It is too early to make any pronouncements on industrial action, but if the company does not respond positively on this and other issues we cannot rule this out."
Terry Pullinger, deputy general secretary postal at the CWU, said: "So we move from shadow boxing to the ring and negotiations will now begin in earnest, and the CWU is fully committed to developing an agreed solution which maintains the pension promise of a wage and dignity in retirement.
"However, any attempt to introduce any unagreed change by the business would be met with an industrial action ballot."
Shares in Royal Mail were down 2% to 454p.

Individual world champion James Guy swam a superb final leg after Dan Wallace, Robbie Renwick and Calum Jarvis had put Britain in contention.
The US were led off by swimming great Ryan Lochte but could not hold on.
"They've always been the best 4x200m team in the world but we really stepped our game up tonight," said Wallace.
"It always makes you confident to have the world champion anchoring your relay but I think we set him up pretty well and it was a great race."
Britain clocked a time of seven minutes 4.33 seconds to claim the nation's first ever men's relay gold at a World Championships, with the US and Australia completing the podium.
The US had won the event, which will be part of the Olympic programme in Rio next year, at the last five World Championships.
Guy, 19, won the individual title on Tuesday and took the British quartet from third to first on the final leg of the relay.
"The way things have been going in racing and training, the work is paying off," said Guy. "The personal bests are finally dropping down."
Britain now stand third in the overall medals table behind the United States and Australia with five gold, one silver and two bronze medals after six of eight days in Kazan.
In Friday's other events, Andrew Willis finished fourth in the men's 200m breaststroke but Lizzie Simmonds failed to progress in the 200m backstroke.
Ben Proud and Fran Halsall qualified for Saturday's 50m freestyle and butterfly finals respectively.

The incident was said to have happened as Mr Galloway gave a talk in the university's New King's building.
He tweeted that he had an "unknown substance in my eyes and lungs" but added later that he was "feeling better... just needed a good shower".
Police confirmed they had been called to a disturbance at 19:45.
Mr Galloway, who is from Dundee, also tweeted: "Someone calling themselves "Trans" and an "anarchist" led a five person attack on me on the platform at Aberdeen University. I continued."
He also said on Twitter that his pregnant wife had leapt forward to defend him.
A Police Scotland spokesman said officers had attended High Street in Old Aberdeen following reports of a disturbance.
"Inquiries are at an early stage and no further details are available at this  time," he said.
Royal Mail has begun a consultation over changes to its pension scheme amid threats of strike action from unions.
[NEXT_CONCEPT]
Great Britain's men overhauled the United States in a dramatic 4x200m freestyle relay final to win gold at the World Championships in Russia.
[NEXT_CONCEPT]
Police were called to the University of Aberdeen following reports that a group of protesters had thrown glitter over former MP George Galloway.