Aerospace giants which employ thousands of British workers have called on Theresa May and the European Union to urgently agree a post-Brexit transition deal.
Boeing, which employs 2,200 people in the UK and supports a further 16,500 jobs, said it had already seen airlines exercising Brexit contingency plans.
In written evidence to Britain's Commons Business, Energy and Industrial Strategy Committee's Brexit inquiry, it called for more "clarity" by April "at the latest".
"The regulated nature of the aerospace industry means that Brexit contingency plans typically have a 12-month lead time associated with them," the company's evidence said.
"Boeing has observed some airlines already executing contingency options.
"Boeing would welcome clarity from the UK and the (European) Commission on the future aerospace business environment in the first quarter of next year at the latest; the company feels that this is in the best interests of UK and EU businesses and consumers."
Airbus, which has more than 15,000 employees across more than 25 sites, also called for a "lengthy transition period".
The firm said it needs the time to invest in customs arrangements like electronic border management systems because it needs to move staff and equipment "urgently (within hours)" around the EU.
"To ensure Airbus is not put at a competitive disadvantage by a trading and regulatory cliff edge, it is vital that the UK and EU agree on the terms of a transitional arrangement as soon as possible," the company's evidence added.
The company also warned that UK jobs could be put at risk if British workers are burdened with new visa requirements after Brexit, or if Britain does not secure access to certain research programmes in the event of "no deal".
The firm stressed UK employees need to be able to move across borders at short notice to fix production line issues, and new visa requirements would make them "more complicated to deal with", affecting "the long-term competitiveness of UK sites".
A no deal scenario would mean the UK could not apply for EU research funding programmes, which are of "increasing importance" to Airbus.
"It is vital in the negotiations that the UK secures continued access to these programmes to avoid highly skilled UK jobs and research moving to EU member states," the company said.
"Not reaching a deal with the EU would have significant consequences for Airbus and its supply chain. Airbus believes that no deal would potentially reduce the UK's influence on regulatory matters, damage the UK's work advancing new technology and, in turn, creating high value jobs.
"Therefore, Airbus relies on a fully-integrated relationship which secures the access to, skills, R&D and influence that underpins its global competitiveness and rising productivity."
Meanwhile, committee chair Rachel Reeves highlighted a warning in the evidence from ADS Group, the national trade association for the sector, that increased customs checks could result in £1.5bn (€1.68bn) added costs.
The Labour MP said: "That is extremely worrying and risks making our aerospace industry less competitive and driving up costs at a crucial time for our economy.
"The extra costs could have a negative impact on future investment decisions. Our committee is keen to hear from the industry what it wants from the Government to ease their uncertainties and ensure the smoothest possible transition as we leave the EU."