David Laws, 33, said to have made a fortune as a merchant banker, was selected by Mr Ashdown's Yeovil constituency as the Liberal Democrat candidate.
The move increased speculation that Mr Ashdown's next appointment, after stepping down from the party leadership, could be to replace Javier Solana at the end of the year as secretary-general of Nato.
Tony Blair was disappointed that Mr Ashdown failed to get the international backing to become the head of the United Nations peace-keeping mission in Kosovo and was said to be pressing Mr Ashdown's candidacy for the more senior role as the international head of Nato.
Aides close to Mr Ashdown dismissed the Whitehall rumours as speculation. "We know nothing about this," his official spokeswoman said.
Mr Ashdown will step down from his Yeovil seat at the next election but Mr Laws will be shadowing the MP in the seat from today.Reuse content