The Prime Minister says his government will provide extra funding for flood-hit areas of Wales if the Welsh Government shows that it needs it.
Boris Johnson also ignored criticism for failing to visit communities suffering damage in the recent storms, saying that he was proud of his government's response.
In the Commons he said that his ministers and officials are "working flat out" with those in Cardiff despite responsibility for flooding being devolved to the Welsh Government.
The First Minister has requested help from the Treasury for major infrastructure projects such as rebuilding bridges.
Several MPs called for the UK Government to ensure that the Welsh Government will get extra funding to deal with the clean-up and repair work needed.
Merthyr Tydfil MP Gerald Jones said to the Prime Minister: "The strength of our union is based on sharing resources in people's time of need but so far not a penny announced by the United Kingdom government will benefit Wales ... The Prime Minister talks the talk on the union but will he today commit a cast iron guarantee that he will provide the major new funding that Wales needs to recover from these floods?"
In response, Boris Johnson said: "Of course it is a devolved matter but nonetheless this government is committed to working flat out with the Welsh devolved administration to ensure that everybody gets the flood relief they need and that cash yes of course it certainly will be passported through."
In an earlier question session in the Commons there was also a demand for the UK Government to hand back money it had taken from the Welsh Government.
Earlier this week the First Minister revealed that recently the Treasury demanded the return of £200m because it had recalculated the Welsh Government's funding.
Labour's Shadow Welsh Secretary, Christina Rees, raised it in the chamber.
She said: "Last month at short notice the Treasury took back £200m from the Welsh government because of recalculations of barnett consequentials. The Prime minister's not bothered to visit flood victims in Wales but could he at least return this money to the Welsh government to help clean up the damage?"
The new Welsh Secretary Simon Hart said such a request hadn't been made during his own meetings with Mark Drakeford but he reiterated that the UK Government will be willing to help.
He said: "We have made it absolutely clear that as soon as the damage is assessed we are ready to assist notwithstanding the fact that this is a devolved responsibility."
The new Welsh Secretary also criticised his opponents for making political points about the flood damage, saying: "If I were a business or an individual affected by the events of the last few days the very last thing that I would be expecting to hear in this house is the politicisation of a very difficult situation."
The trouble with that is, it already is politicised because the money and decisions are made by politicians. And when the flood levels subside, those arguments won't.
During Prime Minister's Questions, the Labour leader Jeremy Corbyn accused Mr Johnson of "hiding" from victims of storm damage.
Mr Corbyn said: "When I visited Pontypridd last week I saw at first hand the damage and destruction these floods have caused to people's lives, homes and businesses. But the Prime Minister was silent, sulking in his grace and favour mansion at Chevening ...When is the government going to stop hiding and show people that he actually cares?"
Boris Johnson replied by saying he was "very proud" of his government's response.
He said: "We convened the National Flood Response Centre on the 14th and since the flooding began there has been a constant stream of ministerial activity led by my right honourable friends the... Secretary of State for the Environment, the Secretary of State for Communities and Local Government."