The telecommunications boss has insisted it will be "impossible" to completely rid the UK network of the Chinese firm's devices over the next 10 years.
And he said there is likely to be security issues and blackouts amid the upgrade to 5G.
Jansen told UK station BBC Radio 4's 'Today' program: "Huawei has been in the telecoms infrastructure for about 20 years and a big supplier to BT and many others in the UK telecoms industry.
"It is all about timing and balance. So if you want to have no Huawei in the whole of the telecoms infrastructure across the whole of the UK, I think that's impossible to do in under 10 years."
He added: . "At BT and in our discussions with GCHQ, we always take the (view) that security is paramount; it is our number one priority," he continued.
"But we need to make sure that any change of direction doesn't lead to more risk in the short-term - I think that's where the detail really matters. If we get in a situation where things need to go very fast, then we go into a situation where service for 24 million BT Group mobile customers is put into question - outages would be possible.
"Secondly, the security and safety in the short-term could be put at risk - this is really critical here. If you are not able to buy or transact with Huawei that would mean you wouldn't be able to get software upgrades if you take it to its specificity."
It's believed that culture secretary, Oliver Dowden, is to comment on the country's future business with Huawei in the UK this week.
Dowden already warned that the US blacklisting the tech giant could have implications for the firm in regards to its future in the UK.