April 23 (Reuters) - Some shippers on Canada's Trans

Mountain expansion project are raising concerns that the

long-delayed oil pipeline will not be fully in service by its

projected start date of May 1, according to a letter to the

Canada Energy Regulator on Tuesday.

Deliveries for all shippers will be subject to the expanded

system's tolls and tariffs from that date, Trans Mountain Corp

told Reuters in an email, adding that line fill on the expanded

pipeline will be completed in early May.

But in a letter to the Canada Energy Regulator (CER),

shipper Suncor Energy ( SU ) said reasonable questions remain

over whether Trans Mountain will be able to deliver contracted

crude volumes from May 1 given some sections of the pipeline are

still awaiting leave to open from regulators.

As a result shippers are concerned about the obligation to

pay tolls from the start of next month, said the letter filed by

Suncor's legal counsel on behalf of other shippers including BP

and Marathon Petroleum ( MPC ).

"While it is possible that Trans Mountain might be able to

complete the physical construction of Expansion facilities by

May 1, 2024, there appears to be a real likelihood that those

facilities will not be capable of providing Firm Service at that

time," said the letter, which appeared on the CER website.

The C$34-billion ($24.81 billion) project, bought by the

Canadian government in 2018 to ensure it went ahead, will carry

an extra 600,000 barrels per day (bpd) of oil from Alberta to

Canada's Pacific coast.

It has struggled with years of regulatory delays and cost

overruns and Canadian oil producers are keenly anticipating its

start-up, which will open up access to export markets on the

U.S. West Coast and Asia and should narrow the price discount on

Canadian heavy crude versus U.S. benchmark oil.

However a number of contracted shippers are locked in

dispute with Trans Mountain over tolls on the expanded system,

citing concerns about significant cost increases.

Trans Mountain is facing a number of complaints from its

shippers, who have contracted 80% of the expanded pipeline's

volume.

In a separate filing on April 12, Canadian Natural Resources

Ltd ( CNQ ), supported by Suncor and Imperial Oil ( IMO ),

wrote to the CER arguing that the vapor pressure limit on the

expanded pipeline is too high and would hurt the sales price of

the crude.

In its email on Tuesday, Trans Mountain also said the first

ship carrying crude from the pipeline expansion is expected to

load in the second half of May.

Westridge Marine Terminal in the Port of Vancouver, where

the pipeline terminates, will have three berths able to load

vessels with oil, Trans Mountain said. The dock has a maximum

capacity of 630,000 bpd, or 34 partially laden Aframax-sized

tankers a month.

"On average, we anticipate one empty tanker in, one

partially laden tanker out every day with variability throughout

the year," a Trans Mountain spokesperson said.

($1 = 1.3660 Canadian dollars)

($1 = 1.3704 Canadian dollars)