Boris Johnson has likened the challenge of avoiding a hard border in Northern Ireland to the boundaries between different boroughs of London.

The foreign secretary said it was a "very relevant comparison" because money was "invisibly" taken from people travelling between Camden and Westminster when he was London mayor.

London's congestion zone charge does not involve manual checks.

Labour said his comments were "typically facile and tactless".

The future of the Irish border after Brexit has been a key sticking point in talks so far.

The UK plans to leave the EU's customs union but wants to avoid border posts and physical checks between Northern Ireland and the Republic.

Mr Johnson, a former London mayor, told BBC Radio 4's Today programme there were "all sorts of things you can do" to achieve this.

"We think that we can have very efficient facilitation systems to make sure that there's no need for a hard border, excessive checks at the frontier between Northern Ireland and the Republic," he said.

"There's no border between Islington or Camden and Westminster... but when I was mayor of London we anaesthetically and invisibly took hundreds of millions of pounds from the accounts of people travelling between those two boroughs without any need for border checks whatever."

He added: "It's a very relevant comparison because there's all sorts of scope for pre-booking, electronic checks, all sorts of things that you can do to obviate the need for a hard border to allow us to come out of the customs union, take back control of our trade policy and do trade deals."

But Labour's shadow Northern Ireland secretary Owen Smith dismissed the comments as "ludicrous".

And he accused Mr Johnson of "glibly dismissing the concerns of thousand of families and businesses who live and work along the border", adding that his remark "insults the intelligence of all who are worrying about how to resolve the border question after Brexit".

Virtual border?

Irish MEP Mairead McGuinness, vice president of the European Parliament, also questioned the analogy. "The UK is a different country than the Republic of Ireland ... and therefore the comparison doesn't quite fit," she told BBC2's Daily Politics.

But DUP MP Sammy Wilson disagreed, saying that a virtual border already exists.

"There's a different tax regime in Northern Ireland than the Irish Republic - VAT, excise duty is different - yet billions of pounds worth of goods cross that border, taxes are paid and not a lorry is stopped to check the goods because through virtual methods, through IT, through electronic invoicing, those taxes are collected by both the Irish government and the British government," he said.

"If it can work on that basis - it can work after we leave the EU."

The first draft of a legally-binding agreement between the UK and the EU, due to be published on Wednesday, is expected to address the issue of Northern Ireland.

The Dublin government says the option of Northern Ireland staying in full regulatory alignment with the European Union after Brexit should be included as a "backstop" - but this is opposed by some Conservative MPs and the Democratic Unionist Party.

Meanwhile former World Trade Organisation chief Pascal Lamy argued that whatever Brexit option was chosen "will necessitate a border" between Northern Ireland and the Republic, because checks will have to be carried out on goods and people.

Instead he suggested to the Commons Brexit committee a "Macau option" for Northern Ireland.

"You should think about giving to Northern Ireland the same autonomous trade capacity that China has given to Macau, which doesn't mean that Macau doesn't belong to China," he said.