UK Government officials remain "confident" they will make sufficient progress by October to ensure Brexit trade talks begin, amid reports Cabinet ministers believe they could be delayed.
The UK's Department for Exiting the European Union added Brexit Secretary David Davis also expects both the UK and EU to have a "dynamic and flexible approach" to each round of the negotiations.
But Cabinet ministers involved in the Brexit talks are said to have privately indicated they think progress to the second phase - focused on a post-Brexit EU trade deal - may not happen until Christmas.
Sky News cited sources as suggesting that next month's federal elections in Germany, including agreeing who forms the new government, could have an impact on the process.
Philip Hammond earlier this month delivered a less-than-certain verdict that EU trade talks would begin this autumn.
Speaking in Brazil, the British Chancellor said there was "hope" from the UK that negotiations over future relations with the EU would begin in the autumn.
His assessment contrasted with a more upbeat tone adopted by Mr Davis.
Their remarks came after reports suggested EU chief negotiator Michel Barnier believes the second phase of talks would be delayed by two months to December because of disagreements over how much the UK owes the bloc.
Citizens' rights also need to be resolved in the initial phase, with disagreements emerging between the two sides.
Around 3.2 million EU citizens live in the UK, while more than a million British expats reside in other EU member states.
A Department for Exiting the European Union spokeswoman, in response to suggestions the trade talks could be delayed, said: "As the Secretary of State has said, it is important that both sides demonstrate a dynamic and flexible approach to each round of the negotiations.
"Government officials are working at pace and we are confident we will have made sufficient progress by October to advance the talks to the next phase."
The UK Government is publishing papers outlining its position on different topics, including the customs union, which it hopes will ensure the talks progress.
Labour MP Peter Kyle, a supporter of the pro-EU Open Britain group, said: "The Government wasted months after triggering Article 50 before starting negotiations and now we might not get to the meat of them before the end of the year.
"Ministers vowed to negotiate an agreement that delivers the 'exact same benefits' as single market and customs union membership, yet the clock is ticking and they could not be further from achieving that already unrealistic goal.
"To protect our economy, the Government should put single market and customs union membership back on the table as the best and quickest option for our future trade with the EU."
The UK's Home Office is also developing plans which would ensure EU citizens are free to travel and live in Britain post-Brexit, according to The Times.
The newspaper said the proposals, which are not agreed across government nor finalised, suggest EU nationals would not be required to apply for a work visa to visit Britain to look for a job.
It also said the Government would seek to limit the number of people migrating to work in the UK via a system of permits, which would vary for different sectors.
A Home Office spokeswoman said: "Proposals for the future immigration system for EU citizens will be published in due course."