Boris Johnson has said it should be possible to "run a better Tube system", fuelling the row about why carriages are packed during the coronavirus pandemic.

The Prime Minister told MPs that more London Underground trains should be running, despite Mayor of London Sadiq Khan insisting services are reduced due to staff being off sick or self-isolating.

The Tube is operating at around 50% of its normal capacity during peak times, leading to overcrowding as many people continue to travel despite pleas for non-essential journeys to be avoided.

NHS workers are among those who have complained that packed carriages are putting their health at risk.

Mr Johnson said: "My own view is that we should be able to run a better Tube system at the moment, we should be able to get more Tubes on the line."

He claimed "I do not wish in any way to cast aspersions on what is going on at TfL (Transport for London)", and pledged to give the mayor "every support" to get through "what seems to me to be his present logistical difficulties".

The Prime Minister's comments come a day after Health Secretary Matt Hancock said there is "no good reason" why timetables have been cut to the extent they have.

Mr Khan said services have been reduced because nearly a third of TfL staff are off sick or self-isolating, including train drivers and control centre workers.

He went on: "TfL will do everything possible to continue safely running a basic service for key workers, including our amazing NHS staff, but if the number of TfL staff off sick or self-isolating continues to rise - as we sadly expect it will - we will have no choice but to reduce services further."

Crowd control measures have been introduced, and British Transport Police officers are patrolling stations to remind passengers that only essential travel is permitted.

The mayor added that "too many of the people using TfL services at the busiest times work in construction", as he reiterated his call for the Government to ban non-safety construction work.

Communities Secretary Robert Jenrick insisted construction workers should continue to go to work amid the coronavirus crisis if social distancing and other safety precautions can be met.

The Cabinet minister told BBC Breakfast: "If you or your employer believes you can continue to operate safely within those guidelines then it's sensible for you to do so."

House-builders Bellway and Persimmon are shutting construction sites despite being allowed to stay open amid the lockdown to help protect workers from coronavirus.

Mr Khan said TfL's early-morning data suggests passenger numbers were down by a third on Wednesday compared with Tuesday. They were already reduced by 88% on Tuesday compared with the same day last year.

But some carriages remained packed. Shortly before 7am, a senior nurse named Danny posted on Twitter: "Another busy tube. Can we not stagger people's start times so we aren't all squashed on the same tube! This is unsafe and not fair!"

Barry Trimble, who said his work involves ensuring cancer patients receive chemotherapy, posted: "The Tube is packed, with social distancing impossible. We need more people to stay at home and more trains running in morning and evening peak."