Robert Downey Jr. welcomed Tom Hiddleston to Instagram by poking fun at his romance with Taylor Swift.

Robert Downey Jr.

Robert Downey Jr.

The 'Captain America: Civil War' star appeared to make a joke about 'The Night Manager' actor's whirlwind relationship with the 'Shake It Off' hitmaker.

Captioning a picture of Tom wearing an 'I <3 T.S.' shirt, he wrote on the photo sharing site: "Join me in welcoming the biggest T. Stark fan of them all to Instagram! @twhiddleston (sic)"

Meanwhile, the 35-year-old actor shocked fans on Tuesday (09.08.16) when he signed up to Instagram and shared his first photo, a picture from the set of 'Thor: Ragnarok'.

Sharing a photograph of himself dressed as Loki, he simply wrote: "He's back!"

And Tom is thrilled to be reprising his role in the Marvel films and likened it to "seeing an old friend again".

He said previously: "It's amazing to be back, amazing. I've only shot for about a week but it's so much fun, because I haven't played the character for three and a half years and the script is brilliant, [director] Taika Waititi is brilliant. I've shot a couple of scenes and it's just the best fun. It is like seeing an old friend again."

Things are thought to be going well for Tom and Taylor, with 'The Avengers' star recently insisting he isn't bothered about the increase in focus on his personal life.

He shared: "I don't know, it comes down to being authentic. Everything you do you have to make sure you truly believe in it and as long as you know that, it doesn't matter what anyone else says about it because the nature of being a public figure is that everyone will have an opinion about anything you do, and as long as you know why you've done something and you've committed to it with authenticity then you're OK."