The two companies have been in talks for more than six months to combine their UK networks - the country's third and fourth largest - and an agreement had been expected imminently.

Sixt, however, signalled there were still issues to iron out.

"It is probable as speculated that we will reach an understanding with our good friends at Vodafone," he told investors after a trading update on Tuesday.

"Although I would say they (Vodafone) are extremely difficult to draw a conclusion with on the one hand, but on the other hand they are, in the end, very good partners."

"So the first part makes me a little bit more skeptical but the second part makes me a little bit more optimistic," he added.

Hutchison's senior leadership met British government officials in March to seek political support for the deal, which will be subject to intense regulatory scrutiny, sources have told Reuters.

Hutchison's co-managing director Canning Fok, who said in March the talks were in the final stages, has also met Vodafone's Chief Executive Margherita Della Valle in a push to get the deal through, sources have said.

(Reporting by Paul Sandle, Editing by Sarah Young)