Are packed structs portable?

I have some code on a Cortex-M4 microcontroller and'd like to communicate with a PC using a binary protocol. Currently, I'm using packed structs using the GCC-specific packed attribute.

Here is a rough outline:

struct Sensor1Telemetry {
    int16_t temperature;
    uint32_t timestamp;
    uint16_t voltageMv;
    // etc...
} __attribute__((__packed__));

struct TelemetryPacket {
    Sensor1Telemetry tele1;
    Sensor2Telemetry tele2;
    // etc...
} __attribute__((__packed__));

My question is:

• Assuming that I use the exact same definition for the TelemetryPacket struct on the MCU and the client app, will the above code be portable accross multiple platforms? (I'm interested in x86 and x86_64, and need it to run on Windows, Linux and OS X.)
• Do other compilers support packed structs with the same memory layout? With what syntax?

EDIT:

• Yes, I know packed structs are non-standard, but they seem useful enough to consider using them.
• I'm interested in both C and C++, although I don't think GCC would handle them differently.
• These structs are not inherited and don't inherit anything.
• These structs only contain fixed-size integer fields, and other similar packed structs. (I've been burned by floats before...)