The stake could fetch around 300 million pounds and may attract pension funds, infrastructure funds and sovereign wealth funds, according to the sources, who are familiar with the situation.

In December, Britain identified its share of the Eurostar cross-channel rail link as one of a number of government assets which could be sold off by 2020 to raise money for the public purse. Bankers had then been invited to pitch and were expecting the government to appoint a financial adviser in the first quarter of this year.

"The Government has never intended to retain its 40% stake in Eurostar. It is seeking to appoint advisers to support our assessment as to whether a sale, in the current climate, of our share in Eurostar would deliver value for money for the UK taxpayer.", said a spokesperson for the Department for Transport (DfT).

Alongside the British government, French state-owned rail operator SNCF holds 55 percent of the company and the Belgian government owns the remaining 5 percent.

A spokesperson for SNCF said that the French company will monitor the British government's intention regarding its 40 percent in Eurostar as soon as it becomes public. The person added that SNCF will remain Eurostar's main industrial partner with 55 percent of the company and also has a pre-emption right on any change in ownership.

An official announcement on the appointment is expected within the next few days, one of the sources said.

Several sector bankers said they didn't expect SNCF to buy from the UK government the remaining stake in Eurostar.

UBS declined to comment.

The sale is part of the British government's plans to privatise 20 billion-pounds of state assets in the next six years and to boost private-sector infrastructure investment.

Established in 1994, the high speed rail service transported more than 10 million passengers in 2013 for the first time in its history, with revenues rising 7 percent year-on-year to 857 million pounds, it said on Wednesday.

Infrastructure projects attract insurers, pension funds and infrastructure funds because they offer the prospect of an inflation-linked return, often with a government guarantee, that helps them meet life insurance and pension liabilities.

(Reporting by Sophie Sassard; Editing by Kirstin Ridley and Elaine Hardcastle)

By Sophie Sassard and Anjuli Davies