Royal Dutch Shell's chief executive, Ben van Beurden, has told investors that Britain's decision to exit the European Union could slow its $30 billion asset sale plan, especially in the North Sea which had struggled to attract buyers for years.

The comment, made during an investor and analyst event at the Wimbledon tennis tournament this week, came as Shell mandated Bank of America Merrill Lynch to find buyers for several key assets in the North Sea, including its stake in the lucrative Buzzard oilfield, hoping the sale would raise at least $2 billion.

The Anglo-Dutch oil major had previously targeted wrapping up the disposal of dozens of assets around the world by roughly 2018 to help fund its $54 billion acquisition of rival BG, which it completed in February.

Chief Financial Officer Simon Henry had previously indicated the divestment programme could take longer than the initial time frame.

But van Beurden said on Wednesday the uncertainty that has engulfed global markets following Britain's Brexit vote on June 23 was set to be an obstacle for the programme, according to two investors who were at the Wimbledon event.

"Ben said that post-Brexit, the disposals could take more than three years to complete," one source told Reuters, asking not to be identified because the event, hosted by Shell, was not open to the public.

The 58-year-old Dutchman, who has stated his ambition to make Shell the best oil company investment ahead of ExxonMobil , said Brexit "will make it more difficult to execute disposals", particularly in the North Sea, according to a second person who spoke to him.

A Shell spokesman said in response that "there has been no change to the previous statements we made on the three-year, $30 billion divestment programme".

BAML also declined to comment.

The company said in June it wanted to exit 10 countries after merging with BG to sharpen its focus on gas production globally and deepwater exploration in Brazil.

Sales in the North Sea, an ageing oil basin that has seen production falling since the late 1990s, have been seen as particularly difficult following the halving of oil prices over the past two years.

A possible Scottish independence drive could further cloud the outlook for the North Sea oil industry and stall mergers and acquisitions in the region due to a lack of confidence in future regulations and taxes, one of the sources explained.

Shell's shares, together with those of British rival BP, have nevertheless made strong gains since the Brexit vote as the weaker British pound and their dollar dividends attract investors.

Shell has in recent days put on the block a package of oil and gas assets in the North Sea including rapidly depleting fields as well as new developments, three banking sources told Reuters.

The company is offering potential buyers a bundle of assets including BG's non-operating stake in Buzzard north of Aberdeen, a relatively new field that feeds into the global Brent oil benchmark, the sources said.

It is also offering a share in Shell's 55 percent holding in the BP-operated Schiehallion oilfield development some 180 km west of the Shetland Islands.

Other assets include the Nelson, Armada, Everest, Lomond and J Block fields, and Shell's stake in the Statoil-led Bressay development.

Deal activity for oil and gas production assets, known as upstream, has slowed significantly following the oil price crash as buyers' and sellers' views on the future oil price diverge.

North Sea M&A activity has been particularly slow because of huge costs associated with ageing fields' clean-up costs.-Reuters